This study is based on a 10% random sample of medical certifications of birth in Quebec in 1970-71 and on additional information obtained from perinatal death certificates and birth registrations. Smoking in pregnancy significantly reduced the average birth weight after 35 weeks of gestation. It increased the risk of perinatal death by 24%. This increase was concentrated in the age groups under 25 and 35 +, in the parity groups 0 and 4 + and among women with less than 12 years of schooling. The proportion of mothers smoking during pregnancy was 43.2% and ranged as high as 62.2% among those under 20 with less than eight years of schooling. These depressingly high percentages imply that new educational approaches aimed particularly at high-risk groups should be developed and evaluated.