Risk factors for sudden infant death syndrome (SIDS) were examined in a prospective study based on Swedish births between 1983 and 1985. All infants surviving the first week of life were included (279,938). The overall rate of SIDS was 0.7 per 1,000 first week survivors. Elevated relative risks were associated with low maternal age, multiparity, maternal smoking, and male infants. Smoking doubled the risk and a clear dose-response relation by amount smoked was observed. Maternal smoking also seemed to influence the time of death, as infants of smokers died at an earlier age. In countries like Sweden, smoking may be the single most important preventable risk factor for sudden infant death syndrome.