Clinical and postmortem findings on 126 consecutive cases of sudden infant deaths occurring in Metropolitan Toronto over a 42-month period were reviewed and classified. Of these cases, 101 were diagnosed as sudden infant death syndrome (SIDS), the other 25 as due to other causes. The SIDS cases were classified into three groups: A-1 (39%), infants born at term who died between 2 weeks and 6 months of age and in whom clinical and autopsy findings were negative; A-2 (4%), same as A-1, except that the infants were born prematurely ( 10 weeks at death), with up to 80% concordance between clinical and pathological indices of respiratory infection. Group B also included cases with minor congenital abnormalities, drug use by mother, and other miscellaneous findings. This report defines the clinicopathological features of SIDS infants and describes the spectrum of findings associated with the syndrome. It is suggested that by applying uniform criteria for definition and classification of SIDS a more reliable data base can be obtained, facilitating research into the causes and pathogenesis of SIDS.