In a multicentre study conducted by thirty-two investigators in ten European, Asian, and Latin-American countries the combination of prazosin 0.5 mg and polythiazide 0.25 mg per tablet was used as the only drug treatment for all grades of hypertension in 551 adult male and female out-patients. After a 2-week placebo period, patients received 1 tablet daily. The dosage was increased, as necessary, by 1 tablet every 2 weeks to a maximum of 4 tablets daily to provide an adequate blood pressure response. Duration of therapy averaged 7.8 weeks. Blood pressure was reduced by an average of 15% systolic and 17% diastolic and the degree of severity of blood pressure improved in 93% of patients. Two-thirds of the patients ended the study with a diastolic blood pressure equal to or less than 90 mm Hg. The dosage at the end of the therapy period averaged 2 tablets of Minizide as a single daily dose. Side-effects were mostly mild or moderate and were typical of those expected for antihypertensive medication. It is concluded that this combination drug is efficacious in the treatment of hypertension and, together with its good toleration profile and ease of administration, will enhance compliance with therapy.