The effects of cigarette smoking and alcohol consumption on the length of gestation were examined in a prospective study of 30,596 pregnant women in northern California. Preterm births (less than 37 weeks' gestation) were 20% more common in women smoking at least one pack of cigarettes per day. This effect was strongest for births occurring before 33 weeks, where the excess was 60%. This excess was not accounted for by differences in maternal age, education, ethnicity, time prenatal care began, drinking during pregnancy, or eight other potential confounding factors. The results indicate a probable effect of smoking on the time of parturition, which is additional to its well-known effect on intrauterine growth retardation. The effect of alcohol consumption on preterm births was also examined, but no consistent trends were found.