A prospective case-control study of sudden infant death syndrome (SIDS) in Norway, Denmark and Sweden between September 1,1992 and August 31,1995 comprised 244 cases and 869 matched controls. After the introduction of risk-intervention campaigns, the SIDS incidence decreased from 2. 3/1000 live births in Norway, 1. 6 in Denmark and 1. 0 in Sweden to 0. 6/1000 or fewer in all the Scandinavian countries in 1995. The decrease paralleled a decline in the prone sleeping position and there was an accompanying parallel fall in total postneonatal mortality in all three countries. Thus, the risk-reducing campaigns for SIDS have been successful not only in Norway and Denmark, starting from relatively high incidences, but also in Sweden, starting from a low incidence. During the study period, a gradual increase was observed for the effects of prone sleeping, smoking and bottle-feeding as risk factors for SIDS.