OBJECTIVE--To investigate long term mortality among women who smoked during pregnancy and those who stopped smoking. DESIGN--A follow up of a geographically defined cohort from 1966 through to 1993. SUBJECTS--11,994 women in northern Finland expected to deliver in 1966, comprising 96% of all women giving birth in the area during that year. Smoking habits were recorded during pregnancy but not later. MAIN OUTCOME MEASURE--Mortality by cause (571 deaths). RESULTS--The mortality ratio adjusted for age, place of residence, years of education and marital status was 2.3 (95% confidence interval 1.8 to 2.8) for the women who smoked during pregnancy and 1.6 (1.1 to 2.2) for those who stopped smoking before the second month of pregnancy, both compared with non-smokers. Among the smokers the relative mortality was higher for typical diseases related to tobacco intake, such as respiratory and oesophageal cancer and diseases of the cardiovascular and digestive organs and also for accidents and suicides. CONCLUSION--The risk of premature death seems to be higher in women who smoke during pregnancy than in other women who smoke. This may be explained either by the low proportion of those who stop later and the high proportion of heavy smokers or by other characteristics of these subjects that increase the risk.