Maternal smoking is the most prevalent risk factor for low birthweight in Canada. This study compared the prevalence of maternal smoking before and during pregnancy from 1983 to 1992. Population-based surveys of 3,296 women during six months in 1983 and 7,940 women during 12 months in 1992 were conducted in Ottawa-Carleton using a self-administered questionnaire completed in the hospital postpartum period. The proportion of women smoking after the first trimester of pregnancy decreased from 28.5% in 1983 to 18.7% in 1992. This difference was due mainly to a reduction in the proportion of women who smoked before pregnancy (37.4% to 26.4%). Another factor was that more women stopped smoking early in pregnancy (23.9% to 29.2%). Gradients in levels of smoking by age, education, marital status and poverty level still exist; however, this is true for the general population. Programs to decrease smoking in pregnancy should continue to focus on reducing smoking among women in general and among those in the preconception and early stages of pregnancy in particular.