Review of the Department of Histopathology autopsy files over a 30 year period from 1962 to 1991 revealed only three cases in which unexpected death occurred in infants under 1 year of age due to massive pulmonary thromboemboli. Predisposing factors included necrotizing enterocolitis with gut perforation and sepsis, a ventriculoatrial shunt and idiopathic arterial calcification. Diagnosis of the latter autosomal recessive condition was only made at autopsy. These cases demonstrate that pulmonary thromboembolism is a possible, although exceedingly rare, cause of sudden infant death, that some predisposing factors are unique to infancy and that the source of the pulmonary thromboembolus may be difficult to determine at autopsy. Massive pulmonary thromboembolism in infancy may point to the presence of other significant, clinically-unsuspected, disorders.