The objective was to analyse differences in the epidemiological pattern of sudden death in infancy during two time periods--the Sudden Infant Death Syndrome (SIDS) 'epidemic': 1984-1989, and the period of rapid decline in the SIDS rate 1990-1996. Sex distribution, age, sleeping position, signs of infection, day of the week and place of death were registered and compared for the two time periods studied in all SIDS cases autopsied at the Institute of Forensic Medicine, Oslo. There were significantly more deaths in the age group under four months in the period 1984-89 than in the second period. Prone sleeping position, signs of infection, death outdoors and during the winter were more frequent during the first period than in the second. These features also were more frequent in the age group under four months than in the older babies during the first period. The shift in the epidemiological pattern after 1990, when the risk factor campaign was launched, indicates that prone sleeping position, cold climate, sleeping outdoors and infections seem to be risk factors that are particularly harmful to the youngest infants.