Because massive pulmonary thromboembolism is a rarely described cause of sudden and unexpected death in the pediatric age group, a 50-year retrospective review of autopsy cases at the Hospital for Sick Children, Toronto, Canada, was conducted to determine the local incidence and clinicopathologic features of such patients. Only 8 cases (0.05%) from a total of approximately 17,500 autopsies were found. The ages ranged from 1 month to 13 years and predisposing factors included recent surgery, congenital heart disease, in-dwelling venous catheters, sepsis, an arteriovenous malformation, occult malignancy, and prolonged immobility. This study confirms that pulmonary thromboembolism is a diagnosis that must be considered in cases of sudden and unexpected death in the pediatric age group. Its incidence, however, appears to be extremely low even in a relatively high-risk population.