Paradoxical embolism is the entry of thrombus from the venous system and right atrium into the left cardiac system through abnormal channels within the heart and/or lungs, causing ischemic stroke and embolism of the cardiac, renal, and peripheral systems, commonly in patients with lower extremity deep vein thrombosis combined with unclosed foramen ovale. The TCD foaming test is an ideal tool for the diagnosis of paradoxical embolism, especially in young and middle-aged patients with cryptogenic stroke. The case we report is a first-episode paradoxical cerebral embolism due to pulmonary arteriovenous fistula. Clinical data The patient was a female, 38 years old, on the 7th day after appendicitis, with sudden onset of left-sided limb loss of motion for 12 hours admitted on November 4, 2014. The patient had no previous risk factors for cerebrovascular disease such as hypertension, smoking, diabetes mellitus, or coronary artery disease. Physical examination: blood pressure 120/70 mm Hg, clear breath sounds in both lungs, heart rate 65 beats/min, cardiac rhythm, no edema in both lower limbs. Consciousness was clear, the left nasolabial fold was shallow, tongue extension was left deviated, the muscle strength of the left limb was grade 4, and the left Bartholomew’s sign was negative. Auxiliary examinations: routine blood, clotting time, blood biochemistry and immunological tests were all within normal limits; cranial MRI suggested a new cerebral infarction in the right temporal lobe; cervical macrovascular ultrasound, transcranial Doppler ultrasound and cranial MRA did not find vascular lesions; deep vein ultrasound of both lower limbs did not find thrombosis; electrocardiogram suggested sinus heart rate and 24-hour ambulatory electrocardiogram did not find paroxysmal arrhythmias; transthoracic cardiac ultrasound did not The TCD foam test, monitoring the left middle cerebral artery, showed a “rain curtain” embolus signal at rest 12 seconds after foam injection; transesophageal cardiac ultrasound did not reveal any right-to-left shunt diseases such as oval foramen failure and atrial septal defect; chest CT showed Right pulmonary arteriovenous fistula. The patient was eventually discharged with embolization of the pulmonary arteriovenous fistula. The use of the paradoxical embolism risk scale can help us to improve the positive diagnosis of paradoxical embolism, and it is crucial to master the skills of diagnosing paradoxical embolism cases. The MRI of the patient’s head suggested that the infarct focus was located in the blood supply area of different branches of the right middle cerebral artery. The patient had no previous risk factors for cerebrovascular disease, no lesions such as atherosclerotic plaque and entrapment were found in the neck vascular ultrasound and MRA, and no possible pathogenic factors were found in the blood examination, so the mechanism of cardiogenic embolism was initially considered. However, the patient’s electrocardiogram and 24-hour ambulatory ECG test did not reveal any arrhythmia, and transthoracic cardiac ultrasound did not reveal organic heart disease. The patient had undergone appendicitis surgery and was bedridden prior to the onset of the disease, with the possibility of associated lower extremity deep vein thrombosis. Based on the above case characteristics and focal manifestations, the patient was considered to have a possible paradoxical embolism. Paradoxical embolism due to a right-to-left shunt is most commonly associated with an unclosed foramen ovale, and the TCD foam test was preferred because it has been shown to be easier to detect smaller foramen ovale than transesophageal ultrasound, and is safe and easy to perform. P-AVF is a rare intrapulmonary vascular malformation in which the pulmonary arteriovenous vessels bypass the intrapulmonary capillary bed and communicate directly with each other. Emboli or bacteria can enter the pulmonary veins directly through the pulmonary arteries and directly into the body circulation, leading to embolic events, of which central nervous system embolism accounts for 30-50%. Therefore, once P-AVF is diagnosed, it should be treated aggressively, and embolization therapy is the main treatment strategy. This case suggests that paradoxical embolization should be given high priority in young and middle-aged patients with cryptogenic stroke. The possibility of pulmonary arteriovenous fistula should be given high priority if a large right-to-left shunt is monitored by the TCD foam test at rest rather than under Vasava respiratory induction, in addition to consideration of a large patent foramen ovale; then noninvasive chest CT should be selected to further exclude P-AVF, and if P-AVF is not detected by chest CT or enhanced CT, transesophageal ultrasound should be selected to exclude patent foramen ovale, and this sequence of ancillary tests use can reduce the risk of examination for patients.