A randomized double-blind investigation of the early, continuous, prophylactic use of hydrochlorothiazide (50 mg daily) was undertaken with 1,030 obstetrical patients. Twenty-eight percent of the patients were teenagers and 39% were nulliparas. Hypokalemia and hyperuricemia were significantly increased in the hydrochlorothiazide group, but the patients were asymptomatic. The total incidence of hypertension was 19.6% in the hydrochlorothiazide group and 21.8% in the placebo group. Preeclampsia occurred in 3% of the former group and in 3.8% of the latter. Excessive weight gain was evident in 13.9% of the placebo group as compared to 7.7% of the hydrochlorothiazide group ( P = 0.002). The average fetal weight and the incidence of prematurity, congenital anomalies, and perinatal mortality were similar in both groups. No complications related to thiazides were encountered in the 1,040 newborns. The data demonstrated that long-term, prophylactic use of hydrochlorothiazide in pregnancy did not alter the incidence of preeclampsia, hypertension, prematurity, congenital anomalies, or perinatal mortality.