To identify features to help paediatricians differentiate between natural and unnatural infant deaths.Clinical features of 81 children judged by criminal and family courts to have been killed by their parents were studied. Health and social service records, court documents, and records from meetings with parents, relatives, and social workers were studied.Initially, 42 children had been certified as dying from sudden infant death syndrome (SIDS), and 29 were given another cause of natural death. In 24 families, more than one child died; 58 died before the age of 6 months and most died in the afternoon or evening. Seventy per cent had experienced unexplained illnesses; over half were admitted to hospital within the previous month, and 15 had been discharged within 24 hours of death. The mother, father, or both were responsible for death in 43, five, and two families, respectively. Most homes were disadvantaged--no regular income, receiving income support--and mothers smoked. Half the perpetrators had a history of somatising or factitious disorder. Death was usually by smothering and 43% of children had bruises, petechiae, or blood on the face.Although certain features are indicative of unnatural infant death, some are also associated with SIDS. Despite the recent reduction in numbers of infants dying suddenly, inadequacies in the assessment of their deaths exist. Until a thorough postmortem examination is combined with evaluation of the history and circumstances of death by an experienced paediatrician, most cases of covert fatal abuse will go undetected. The term SIDS requires revision or abandonment.