We have investigated the hearts from 153 infants found dead in their cots at ages ranging from one month to one year. The deaths occurred over a period of five years (January 1986 to December 1990) and were studied in a center for the study of the sudden infant death syndrome located in Paris. The epidemiological characteristics of this group are:male predominance (sex ratio 1.43), age less than four months (82%), and a predominance of winter deaths. These are the characteristic features ofthe sudden infant death syndrome. Ofthe 143 children studied with the permission of their parents, 24 (16.8%–12 girls and 12 boys) had histological lesions consistentwith myocarditis according to the Dallas criteria. The histological diagnosis of myocarditis is based on the association of cellular necrosis with a mononuclear or mixed inflammatory infiltrate. Cytoplasmic vacuolization, the presence of inflammatory cells in myocytes, and rupture of the cell walls of myocardial fibres are the equivalent histological signs of cellular necrosis. Myocarditis was diversely associated with respiratory, hepatic, muscular, gastrointestinal and/or neurological involvement. Twelve infants had previously been ill. Two died during the course of a serious illness. In only four cases was a viral association demonstrated. This incidence of myocardial involvement, similar to thatdescribed elsewhere in the literature, suggests that myocarditis could be a pathogenic explanation of some deaths thought on general autopsy investigation to be sudden and unexplained.