The safety and efficacy of indoramin and prazosin added to hydrochlorothiazide (HCTZ) were compared in a double-blind trial involving 209 patients with mild to moderately severe essential hypertension. Patients whose supine diastolic blood pressure (SDBP) did not decrease to less than or equal to 90 mm Hg after 6 weeks of HCTZ therapy had indoramin or prazosin added to their regimen. Mean SDBP during 6 months of combination therapy with either regimen decreased by approximately 10 mm Hg from that at the final evaluation during HCTZ therapy (p less than 0.001); differences between the groups were not statistically significant. Mean heart rate was unchanged, whereas mean weight increased (p less than 0.001) above final HCTZ values by approximately 2 kg in both groups. Mean weight increased significantly (p less than 0.01) from baseline values, however, only in the prazosin/HCTZ group. Approximately 95% of the patients in each group had clinically significant decreases in SDBP. Fatigue or tiredness and dizziness were the most commonly reported adverse effects, and their frequencies were not significantly different in the two groups. Cardiac arrhythmias occurred only in patients in the prazosin/HCTZ group and were significantly (p less than 0.05) more frequent than among patients in the indoramin/HCTZ group; less severe adverse experiences, i.e., dry mouth, ejaculatory problems, drowsiness, and sedation, were significantly (p less than 0.05) more frequent in the indoramin/HCTZ group. When added to HCTZ, indoramin and prazosin are equally safe and effective in the treatment of hypertension.