131 cases of the Sudden Infant Death Syndrome (SIDS) among infants born in the Municipality of Copenhagen during 1956--1971 were analysed on the basis of data collected prospectively by the infant health visitors and abstracted from police reports. Compared with controls, a significantly larger number of SIDS infants had been born out of wedlock, were living only with their mother, had parents in a less secure occupation, lived in more crowded, poor-quality dwellings and districts. The "home standard" was lower among the SIDS families, including a lower standard of infant care, a higher percentage of not keeping the appointments with the infant health visitors and a lower mental capacity in the mothers; there was no difference with respect to the mother's physical capacity. More congenital malformations and more cases of asphyxia were found among the SIDS infants, and a significantly larger number of the SIDS infants had been admitted to hospital, with a tendency to being kept there longer. It is concluded that a relationships exists between poor social conditions, increased morbidity, and SIDS. At the same time, it is pointed out that during recent years the differences between cases and controls as regards certain of the social parameters studied have decreased.