We sought to determine the effects of cigarette smoking on menstrual function using prospectively recorded menstrual data in a cohort study of women ages 37-39 years. Eighty-three current smokers and 275 nonsmokers provided menstrual data for analysis. Smoking was associated with decreased duration of bleeding, increased daily amount of bleeding (subjectively scored), and increased duration of dysmenorrhea. These effects were most pronounced in the heaviest smokers. Smoking was not associated with cycle length, but we found some evidence for increased variability of cycle length among heavier smokers. We conclude that cigarette smoking affects menstrual function, most importantly by increasing the duration of dysmenorrhea.