A cross-sectional study of 873 pregnant women attending a prenatal care clinic was undertaken in Pelotas (Southern Brazil), over a one-year period (1989-90) for the purpose of identifying possible risk and prognostic factors for smoking during pregnancy. The prevalence of smoking at the beginning of pregnancy was 40.8%. The smoking habits of the woman's mother and partner, as well as her educational level, were associated with smoking at the beginning of pregnancy. Smoking in the partner was associated with an approximately two-fold increase in the risk of smoking. The smoking interruption rate, until the 15th-22nd gestational week, was of 35.6%. Family income, smoking habits of woman's mother and partner, age at starting, duration and intensity of the woman's habit were all associated with the interruption during pregnancy. The above results were not affected by adjustment for confounding variables made through stratified analysis.