Acute pediatric mesenteric lymphadenitis is a common pediatric disease, mostly viral in nature, often associated with respiratory and intestinal infections. It is common in children under 7 years of age, occurs in winter and spring, and is often complicated by acute upper respiratory tract infections or secondary to intestinal inflammation. The typical presentation is a sore throat and malaise followed by fever, abdominal pain, vomiting, and sometimes diarrhea or constipation after an upper respiratory tract infection. About 20% of children have enlarged lymph nodes in the neck. Abdominal pain is the main symptom, which is variable in nature and can be vague or spasmodic, with milder symptoms in between episodes of pain. The abdominal pain can be anywhere in the abdomen, but the right lower abdomen and periumbilical area are predominant because the lesion mainly invades a group of lymph nodes in the terminal ileum. The most sensitive site of tenderness may be different on each physical examination, with pressure pain near the midline or high, not fixed as in acute appendicitis, and to a lesser extent than in acute appendicitis, with less rebound pain and abdominal muscle tension. Occasionally, small nodule-like masses with pressure pain may be found in the right lower abdomen as enlarged mesenteric lymph nodes. Acute mesenteric lymphadenitis should be considered when the child presents clinically with fever, abdominal pain, vomiting with upper respiratory tract infection, or after an inflammatory bowel disease without abdominal muscle tension. The pathogenesis is mostly due to the very rich lymphatic drainage of the distal ileum, with many lymph nodes in the ileal and large intestinal regions. After upper respiratory tract infection or intestinal infection, viruses, bacteria and their toxins reach the lymph nodes in this area along the blood circulation, causing mesenteric lymphadenitis. Viral infections manifest as mesenteric lymph node hyperplasia, edema, and congestion, but cultures are negative. Mesenteric lymphadenitis caused by Salmonella infection differs from viral lymphadenitis in that the lymph nodes affected by bacteria mostly show acute inflammatory reaction, hemorrhage and necrosis in the lymph nodes, and Salmonella can be isolated from the lymph nodes. The presence of high fever and abdominal pain in children with upper respiratory tract infection or intestinal infection should be considered as a possible complication of acute mesenteric lymphadenitis. Early ultrasound examination of the abdomen can clarify the diagnosis, prevent misdiagnosis of the disease, help to correctly manage the condition, guide the treatment and improve the efficacy. Diagnostic criteria: Combined with domestic and foreign literature, the criteria for determining lymph node enlargement by ultrasound: normal mesenteric lymph nodes are less than 7 mm in diameter; more than 2 lymph nodes are visualized in the same area of mesentery, and lymph nodes with long-axis diameter greater than 10 mm, short-axis diameter greater than 5 mm, and aspect ratio greater than or equal to 2 are considered enlarged; transverse diameter greater than 1.5 CM indicates pathological enlargement. The distribution of blood flow in the lymph nodes: the blood flow signal is distributed along the lymph gates. The main treatment options are: 1. temporary fasting, intravenous rehydration to facilitate intestinal rest, and in mild cases, less food can be taken temporarily; 2. oral Chinese medicine and antiviral therapy are recommended; 3. abdominal physical therapy (such as warm water bags or ultrashort waves given to the nearest doctor) can be given, but if the child has heavy abdominal pain, physical therapy is recommended to be suspended; 4. if the blood test shows signs of infection, oral therapy can be given. If the blood test shows signs of infection, oral or intravenous antibiotics can be administered; 5. But a very small number of children may not improve after the above treatment, or difficult to distinguish from acute appendicitis, surgery is appropriate. In cases caused by Salmonella, if an abscess is formed or symptoms of peritonitis appear, surgical drainage is performed. Some children may be complicated by intussusception and should be observed. Salmonella infection causing gastrointestinal disease is most common in gastroenteritis, and acute mesenteric lymphadenitis has also been reported. Mesenteric lymphadenitis caused by Salmonella infection is different from viral lymphadenitis and is more likely to occur in children or adolescents. Bacterially-infected lymph nodes mostly show acute inflammatory reaction, hemorrhage and necrosis in the lymph nodes, and Salmonella can be isolated from the lymph nodes. The treatment should be conservative first, and if an abscess forms or symptoms of peritonitis appear, then surgical drainage is performed. The prognosis is very good, and most children recover without any specific treatment. Some children have a good prognosis after anti-inflammatory and antiviral treatment. Death is rare and may occur only in the presence of secondary specific bacterial infections (septicemia due to Streptococcus haemolyticus, rupture of lymph nodes and post-rupture abscesses with peritonitis). It is important to note that in children, because of their incomplete immune function, any factor that stimulates their organism may lead to enlarged lymph nodes. The problem of transitional use of antibiotics is more or less present in all hospitals, and the sonographer must be careful when reporting this diagnosis of lymphadenitis, otherwise it will prompt the clinician to use antibiotics even more. The fact is that most children have a functional abdominal pain of a transient nature, which usually does not last more than ten minutes, and everything is normal after the pain has passed, which is mainly related to the fact that children’s gastrointestinal tract is not well developed and their diet is irregular and unreasonable. Most of them can detect enlarged lymph nodes during ultrasound examination when abdominal pain strikes, but they do not always need antibiotics.