The effects of prazosin treatment on blood pressure and diabetic control were assessed in 22 patients with stable non-insulin-dependent diabetes mellitus and hypertension. After an initial six-week baseline period, patients were titrated to optimal therapeutic doses of prazosin (mean daily dose, 12.9 +/- 6.5 mg). Both sitting and standing systolic and diastolic blood pressures were significantly decreased (p = 0.01) with prazosin therapy from a mean of 152/99 mm Hg sitting and 144/99 mm Hg standing to a mean of 139/84 mm Hg and 133/85 mm Hg, respectively, at the end of titration and throughout the 12-week prazosin maintenance therapy period. Seventy-seven percent of patients achieved the goal sitting diastolic blood pressure of 85 mm Hg or less. Total cholesterol, high-density lipoprotein cholesterol, and triglyceride levels were not significantly altered during prazosin therapy compared with baseline measurements. Diabetic control and renal function were maintained during prazosin treatment with no significant changes from baseline noted. No unexpected adverse experiences were reported. In summary, prazosin treatment effectively reduced blood pressure without compromising diabetic control or renal function in this group of hypertensive patients with concomitant diabetes mellitus.