A long-term follow-up study was carried out in 90 patients with moderate to severe hypertension to evaluate the effectiveness of prazosin given alone or as part of combined treatment with other antihypertensive agents. The aim was to obtain and maintain adequate blood pressure control with as few side-effects as possible. Treatment was discontinued during the first year in 11 cases, 9 because of side-effects and 2 for other reasons. The remaining 79 patients were kept under observation for an average period of 5.3 years (range 3 to 6 years) while being treated with prazosin. Of these, 19 had previously suffered serious cardiovascular complications. The results showed that prazosin proved adequate as sole medication in 6 patients during the entire observation period and satisfactory reduction of blood pressure (less than 90 mmHg diastolic) could be achieved and maintained in over 80% of patients with a combination of prazosin and beta-blockers, with supplementary treatment with diuretics in some cases as and when necessary because of fluid retention and consequent loss of efficacy. Dosage was adjusted to individual needs and a striking feature of the results was the decreasing dose requirement, a trend which was most marked after 2 years of treatment. Over-dosage occurred in a few cases for this reason but treatment, in general, was well-tolerated, most side-effects occurring during the first year.