Prazosin administration caused a significant and continuous antihypertensive effect when given as a single agent for 12 months. The daily dose was stabilized after three months at 6.0 mg per day. After 12 months of prazosin treatment, high-density lipoprotein cholesterol increased by 17 percent (p less than 0.005) and the cholesterol ratio increased by 19 percent (p less than 0.05), but total cholesterol was not significantly changed. There were no statistically significant changes in triglycerides, plasma renin activity, and plasma aldosterone concentration following treatment when compared with baseline levels. Prazosin monotherapy is concluded to have favorable effects on serum lipids and can be considered suitable for long-term antihypertensive therapy.