The aim of the investigation has been to describe Sudden Infant Death Syndrome, SIDS, in Sweden, its background, true incidence and epidemiological characteristics, and finally to offer possibilities of prophylaxis. The method used has entailed a retrospective survey of all 1,873 post-perinatal (1-51 weeks) deaths from six birth cohorts in 1973-1977 and 1979. Through comprehensive documentation, 334 deaths have been isolated as SIDS. The result was an incidence of 0.54/1000 live births. Internationally, this incidence is low, but the study could establish that SIDS increased during 1973-1977 and 1979 in Sweden. The epidemiological characteristics shown by the study agree to an extent with American and British investigations, but reported Swedish incidence is lower. An assessment of the total post-perinatal mortality has also been made to demonstrate that there is no communication between SIDS with a uniform diagnosis and other causes of death. Analysis has been binary: in relation to factors in society, and on an individual basis. Geographical or social bases did not affect incidence of SIDS in Sweden, which is remarkable when comparing these findings with other reports on the subject. In regards to individuals, the analysis has been based upon the place where the infant died. Deaths in the infant's own bed showed a clear rise on Saturdays, but the incidence here remained constant over the period studied. Deaths in bed with one/two adults showed epidemiological findings which indicated disadvantageous family and social factors in many cases; this group increased from year to year. Deaths outdoors were common in winter; a certain increase towards the end of the week as well as from year to year was indicated. No social strain was shown. Deaths during car rides clearly increased from year to year, but showed no particular socio-familial profile nor weekday variation. Findings analyzing location of death have indicated that the different places have their own epidemiological characteristics distribution. Analyses of this sort are scarce. There were no certain indications of SIDS occurring more often in socially disadvantageous circumstances: 4/5 of the SIDS families had no social strain. The study has also included medico-historical notes, e.g. 'over-laying' and a description of epidemiological/pathological and forensic aspects of sudden infant death from Stockholm during the 1840s. SIDS means that neither anamnesis nor a thorough autopsy can explain the death. SIDS may be considered a dysfunction syndrome, in which endogenous and exogenous factors act together and silently lead to death.(ABSTRACT TRUNCATED AT 400 WORDS)