Child A, female, 10 years old, was admitted to the hospital with cyanosis of the lips and mouth (according to the New York Heart Association classification). Cardiac auscultation revealed a grade 2/6 systolic heart murmur, chest radiograph showed increased lung texture, enlarged heart, and a cardiothoracic ratio of 0.63. The electrocardiogram showed left ventricular hypertension with abnormal ST-T in multiple leads. Cardiac ultrasound showed enlarged left ventricle (left ventricular systolic internal diameter 6.5 cm), enlarged left atrium (4.2 cm), and left ventricular ejection fraction 28%. Child B, male, 11 months, was admitted with breath-holding and cough. On examination, a grade 2/6 systolic heart murmur and hepatomegaly (2.5 cm below the rib cage) could be detected. The chest radiograph, electrocardiogram and cardiac ultrasound showed the same as the previous child, but the chest radiograph showed a cardiothoracic ratio of 0.71, a left ventricular systolic internal diameter of 3.4 cm and an ejection fraction of 18%, along with hepatic impairment (glutathione aminotransferase: 2184 u/l, glutathione aminotransferase: 2332 u/L). The diagnosis of dilated cardiomyopathy was clear according to the World Health Organization/International Society of Cardiology Criteria Consortium diagnostic criteria. The child was initially treated with digoxin, diuretics, captopril, anticoagulants and high-dose gammaglobulin (2G/KG) after admission to the hospital. After the above conventional treatment, the child’s clinical symptoms improved. Three years after the discharge of child A, the left ventricular internal diameter did not decrease significantly, the left ventricular ejection fraction was <50%, and the cardiac function was still IIC class III. With the family's consent, umbilical cord mesenchymal cell intramuscular injection therapy was used. This clinical trial was registered with the NIH (registration website: clinicaltrials.gov; registration number: No. NT01219452). After the injection, the children were kept for 24 hours without complications related to injection local bleeding or fever. The second injection was performed 1 month after the first injection, and the injection method and dose were the same as the first injection. After the stem cell treatment was performed, the child returned to the hospital for regular review, and the child's clinical symptoms improved significantly and his cardiac function improved. At 3 months after stem cell treatment, the child's heart function improved to grade 2, and at 9 months, the child's heart function improved to grade 1, and the quality of life improved significantly. At the follow-up until 1 year, the clinical symptoms of the child disappeared, and the reexamination of cardiac ultrasound showed that the left ventricular internal diameter was significantly reduced and the ejection fraction was significantly improved (Figure 1); the chest X-ray showed that the heart was reduced and the cardiothoracic ratio was normal (Figure 2). At the first follow-up visit three months after treatment, child B showed clinical improvement, normal liver function, and an increase in left ventricular ejection fraction to 35%; 12 months after treatment, the left ventricular ejection fraction increased to 59%; 16 months after treatment, the left ventricular ejection fraction increased to 59% and the left ventricular thrombus decreased to 1.0 cm; 22 months after treatment, the left ventricular ejection fraction increased to 62% and the left ventricular thrombus decreased to The clinical manifestations included progressive heart failure, arrhythmia, thrombosis, and sudden death; chest radiography showed a cardiothoracic ratio of >0.5, and cardiac ultrasound showed dilated left ventricle (left ventricular end-systolic internal diameter >2.7 cm/m2) and cardiac systolic dysfunction (ejection fraction <2.7 cm/m2). Dysfunction (ejection fraction <50%) 1. According to the above criteria, the diagnosis of dilated cardiomyopathy was clear in both children after excluding secondary cardiomyopathy and Creutzfeldt-Jakob disease. Treatment Dilated cardiomyopathy is mainly treated symptomatically, including anti-heart failure drugs, vasodilators, β-blockers, etc.2 In this case, the clinical symptoms of the two children improved after drug therapy alone, but the cardiac function was always in class II-III, with insignificant left ventricular internal diameter reduction and low left ventricular ejection fraction (<50% in the first child and <30% in the second. Based on the child's medical history and current condition, the family agreed to stem cell therapy. Human umbilical cord MSCs have multidirectional differentiation potential and can be differentiated into bone cells, cardiomyocytes, chondrocytes, adipocytes and so on. Unlike autologous bone marrow MSCs, umbilical cord MSCs are widely sourced, non-invasive to obtain, and can be expanded in vitro in culture (cells can be stored in a frozen solution of fetal bovine serum and dimethyl sulfoxide in a -80°C refrigerator for a long time), and maintain low immunogenicity and have immune down-regulation function, so umbilical cord MSCs can be used for allogeneic cell transplantation therapy. The cultured umbilical cord MSCs are free from contamination and death and are safe and reliable as allogeneic transplantation cells. The most ideal way of stem cell transplantation is simple, less invasive, less side effects of the body and can maximize the effect of stem cells. Combining the theoretical basis and clinical practice, we decided to adopt the method of intramuscular injection of MSCs into the extremities according to the patient's own condition. Before the injection, electrocardiogram, cardiac ultrasound, blood routine, blood biochemistry, myocardial enzymes and hepatitis virus tests were performed. At follow-up after the injection, the ejection fraction and left ventricular diameter of the child's heart improved compared to before, and the child's condition was stable without further heart failure. Preclinical studies found that the mechanism of umbilical cord MSCs for dilated cardiomyopathy includes: 1. Inhibition of myocardial inflammatory response 2. Inhibition of cardiomyocyte apoptosis 3. Stimulation of vascular regeneration 4. Homing to the lesion and direct repair of cardiomyocytes 6 5. HLA-DR negativity and inhibition of allogeneic response generated by lymphocytes. In contrast, intramuscular injection of stem cell therapy may improve cardiac function mainly through paracrine secretion of multiple cytokines. After the stem cell injection, the oral medication was gradually reduced as the child's clinical symptoms improved. After 1 year of follow-up, the children had reached normal levels of cardiac function and daily activities. Conclusion Previous stem cell therapy via coronary or endocardial injection is invasive and can produce scarring at the injection site, arrhythmias, calcifications or small foci of infarction; while most cells remain in the lungs when injected by the intravenous route8,9; experiments have shown that only a small number of cells injected into the animal heart are present at the injection site or become new, beating cardiomyocytes, so stem cells promote vascular regeneration or paracrine growth factor , cytokine pathway becomes more important in clinical treatment. According to the above theory, intramuscular injection of umbilical cord MSCs can be a non-invasive stem cell therapy route. In this case, it was confirmed that intramuscular injection of umbilical cord MSCs was safe and effective in the treatment of dilated cardiomyopathy in children, and it could significantly improve the cardiac function, reduce the left ventricular internal diameter and improve the quality of life of the children. However, the long-term safety and efficacy of stem cell therapy still need longer follow-up and evaluation.