Two hundred twenty-six patients with mild to moderate hypertension were treated with terazosin in an open, multicenter study to determine the drug's long-term efficacy and safety. All patients had previously received terazosin in a short-term study; 66 patients entered from later short-term studies and had experienced a 7 mm Hg decrease in supine diastolic blood pressure in response to terazosin. Terazosin was administered once or twice daily either alone or in combination with a diuretic and/or a beta blocker. Supine systolic and diastolic blood pressures were significantly decreased from baseline during time intervals ending at 90, 180, 360, and 720 days of long-term therapy. No clinically significant trends were observed in pulse rate, clinical laboratory test results, physical examinations, or electrocardiograms. Patients had a tendency toward a slight weight gain. The most common adverse experiences attributed to terazosin monotherapy were dizziness and asthenia (9.7 percent and 6.6 percent, respectively). Adverse experiences were usually of mild or moderate severity. Of the 226 patients who underwent long-term therapy, 29 (13 percent) withdrew because of adverse experiences, and three (1 percent) withdrew because of uncontrolled blood pressure. This study demonstrates that terazosin is effective and safe for long-term treatment of hypertension.