Labetalol and hydrochlorothiazide (HCTZ) were compared for their efficacy in controlling hypertension of blacks in a prospective, double-blind study. Sixty-one adult patients with mild to moderate hypertension (standing diastolic blood pressure greater than or equal to 95 mm Hg and less than or equal to 114 mm Hg) were randomly selected to receive either labetalol 100 mg twice daily (n = 30) or HCTZ 25 mg twice daily (n = 31). The study was divided into two phases: a 4-week placebo run-in phase, during which all previous antihypertensive medication was discontinued, and a 12-week drug treatment phase. Labetalol and HCTZ doses were titrated to 400 mg twice and 50 mg twice daily, respectively, during the first 6 weeks of the drug treatment phase for those patients not achieving blood pressure control (standing diastolic blood pressure less than 90 mm Hg and a decrease of 10 mm Hg from baseline) on initial dosages. By the end of the 12 weeks of drug administration, patients on labetalol experienced a mean decrease of 10 mm Hg in standing diastolic blood pressure compared to a mean decrease of 10.1 mm Hg in patients on HCTZ. No differences were observed between the two treatment groups in reductions of either standing blood pressure or heart rate. While 19 of 30 patients on labetalol (63%) achieved blood pressure control at some point during the study with a mean daily dose of 568 mg, 18 of 31 (58%) HCTZ-treated patients achieved control with a mean daily dose of 72 mg.(ABSTRACT TRUNCATED AT 250 WORDS)