Objective: The object of this study was to examine the association between maternal smoking and hypertension during pregnancy. Study Design: We used data from the Collaborative Perinatal Project, a large prospective cohort study that collected detailed information on blood pressure, proteinuria, smoking, and placental morphologic and histologic characteristics. A total of 9651 healthy primigravid women without chronic hypertension who had been enrolled in the study at the first or second trimester (average 18 weeks’ gestation) and had had ≥3 prenatal visits were included. Gestational hypertension was defined as diastolic blood pressure ≥90 mm Hg on 2 occasions from 24 weeks’ gestation to 2 weeks post partum. Preeclampsia was defined as gestational hypertension plus ≥2 urine samples containing ≥1+ protein according to dipstick measurement during the same gestational period. Results: After we controlled for prepregnancy body mass, age, socioeconomic status, and race, both past smoking and smoking during pregnancy were associated in a dose-response pattern with reduced risks of gestational hypertension and preeclampsia. For women who smoked ≥10 cigarettes/d the relative risks with respect to women who had never smoked were 0.6 (95% confidence interval, 0.4-0.9) for gestational hypertension and 0.5 (95% confidence interval, 0.4-0.7) for preeclampsia. This protective effect was observed both for mild and severe gestational hypertension and for preeclampsia. The more and the longer a woman had smoked previously, the lower was her risk of development of hypertension during pregnancy. This association could not be explained by confounding factors, by changes in placental morphologic or histopathologic characteristics, by maternal net weight gain, or by elevated liver enzyme bioactivity. Conclusion: Smoking is associated with a reduced risk of hypertension during pregnancy. The protective effect appears to continue even after cessation of smoking. Further basic research on this issue is warranted. (Am J Obstet Gynecol 1999;181:1407-13.)