In the investigation of sudden unexpected death in infancy (S.U.D.) the antenatal and immediate postnatal history of the child and associated maternal circumstances have been virtually unexplored. These areas were studied using data gathered in a perinatal mortality study carried out in Ontario. By using rigid criteria, 80 cases of S.U.D. were identified and matched with live controls. Subsequent analysis showed that S.U.D. occurred slightly more often in males than females, and that the majority of deaths occurred under 4 months of age. S.U.D. was shown to be significantly related to prematurity; feeding other than at the breast; low maternal age at time of marriage, first pregnancy and the delivery of the infant under study; delayed first prenatal visits; maternal blood group and cigarette smoking. Confirmation of these findings may allow the construction of profiles of infants particularly at risk and permit the institution of preventive measures. Further prospective studies in this field are required.