We compared cigarette smoking during pregnancy among 574 women who aborted spontaneously (cases) and 320 women with delivery after at least 28 weeks' gestation (controls). Log-linear analysis was used to test the hypothesis that smoking is associated with spontaneous abortion. Several potentially confounding variables--namely, age at last menstrual period and the number of previous pregnancies ending in spontaneous abortion, in induced abortion and in live birth--were controlled in the analysis. Women who had aborted spontaneously reported smoking during pregnancy more often than those with delivery after 28 weeks' gestation: 41% of cases and 28% of controls smoked. The odds ratio for the highly significant association with smoking was 1.8. The association did not vary with age or previous obstetric events. We conclude that smoking during pregnancy is a risk factor for spontaneous abortion.