We report the results of a population-based randomized clinical trial that tested the effectiveness of a prenatal self-help smoking cessation program. The intervention consisted predominantly of printed materials received through the mail. The population (n = 242) consisted of a socioeconomically and ethnically diverse group of pregnant women enrolled in a large health maintenance organization (HMO) who reported they were smoking at the time of their first prenatal visit. Biochemical confirmation of continuous abstinence achieved prior to the 20th completed week of pregnancy and lasting through delivery revealed 22.2 per cent of the women in the eight-week serialized program quit versus 8.6 per cent of controls with usual care. The adjusted odds ratio was 2.80 (95 per cent CI = 1.17, 6.69). We conclude that a low-cost prenatal self-help intervention can significantly affect the public health problem of smoking during pregnancy and its associated risks for maternal and child health.