The incidence of morphologic variations (MVs) was investigated in 180 infants who died unexpectedly and suddenly and in 180 age-matched nonsuddenly deceased infants. The same person (G.M.) performed a full postmortem and histologic examination on all of the infants, none of whom had significant malformations. The test group (TG) consisted of 146 infants whose death was unexplained, whereas group 3 (G3) consisted of 34 infants with adequately determined causes of death. The principal findings in the control group were prematurity, inflammatory and metabolic diseases, tumors, and traumatic lesions. MVs were present in 62% of the TG infants, in 47% of the G3 children, and in 22% of the controls. The differences between the TG and G3 on the one hand and the controls on the other are statistically significant (p < 0.001 and 0.05, respectively). In the TG, anomalies were twice, dysplasias three times, and dysmorphisms four times as common as in controls; in G3, four times as many dysplasias and dystropics were found as in controls. In the TG, 23% of the MVs were multiple as compared with 9% in G3 and 5% in controls. The pattern of distribution was not confined to certain organs. Among the relative high percentage of dysplasias, the large number of neuroblastomas is striking.