This double-blind study was conducted to investigate the efficacy, safety and tolerability of three dose levels of moexipril in comparison to placebo as addon therapy to hydrochlorothiazide (HCTZ) in patients with moderate to severe hypertension. Two hundred patients who did not respond adequately to a 4-week monotherapy with HCTZ – sitting diastolic blood pressure between 95 and 114 mm Hg – entered the 8-week double-blind period. Patients were randomized to once daily placebo or moexipril 3.75, 7.5 or 15 mg as add-on therapy to open-label HCTZ 25 mg. At biweekly visits, blood pressure and heart rate measurements were obtained and the occurrence of adverse experiences was documented. At the 8-week endpoint, adjusted mean reductions from baseline were significantly (p = 0.003) greater in patients receiving moexipril 3.75, 7.5 and 15 mg compared to placebo (-8.4, -8.8 and -8.9 vs. -4.6 mm Hg). No significant differences between the three dose levels of moexipril could be observed. Moexipril was generally well tolerated. The most frequently reported adverse events for moexipril and placebo were headache, flu syndrome and dizziness (6, 7, 5 vs. 4, 0, 4%). The results indicate that the combination of moexipril and HCTZ is a clinically valuable combination in the treatment of patients with moderate to severe hypertension.