Sudden infant death [SID] is defined as the "sudden death of an infant under one year of age that remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene and the review of the clinical history". This definition, given by the National Institute of Child Health and Human Development in the USA. recognized SID as a diagnostic entity, however, SID remains a pathological diagnosis of exclusion. SID-cases are divided into three categories: group 1 in which autopsy does not reveal any cause of death, group 2 in which postmortem findings are not sufficient to be the cause of death; group 3 in which the death is adequately explained, is classified as "non-SID". The purpose of the study was to analyse the situation about SID over a 25-year period, from 1969 through 1993, looking for changes in frequency and developmental trends. The study includes 364 SID-infants which were examined according to a standard method. Histological, microbiological, virological and immunological studies were performed and data of the death scene investigation and of the clinical history were collected, 284 infants who had died due to clinical diseases served as one control group, a second group consisted of 6397 newborns of the city of Zürich. Of the SID-infants, almost every third infant [29%] was grouped in group 1, more than half of the infants [52%] with signs of mild infections in group 2, whereas almost every fifth child [19%] was grouped in group 3. With regard to epidemiological parameters the peak of mortality was found at about three months of age; the percentage of boys and twins, of third and later born infants was higher compared to the general population. SID probands were born twice as often in October as in March. The percentage of mothers of first, second and third born SID-infants at an age younger than 20, 22 and 24 years was higher compared to the general population. All these parameters were also found in group 3. The risk of familial recurrence in group 1 and group 2 was 0.8%. Between 1969 and 1993 the rate of SID deaths dropped from 1.1 to 0.9 per thousand live births, whereas SID as a percentage of postneonatal mortality increased to 34 per cent.