A drug surveillance study was performed to determine the tolerance and safety of quinapril in the treatment of patients with stage 1 or 2 hypertension. The trial was noncomparative, open-label, uncontrolled, and nonrandomized. Patients with secondary hypertension, heart failure, other heart diseases, and other serious conditions were excluded. After a washout period of 2 weeks, 752 patients (316 men and 436 women) with diastolic blood pressure (DBP) between 90 and 115 mm Hg and systolic blood pressure (SBP) between 140 and 200 mm Hg were entered into the treatment phase. The mean age of patients (+/- SD) was 53.1 +/- 11.4 years. Patients initially received 10 mg/d quinapril for 4 weeks. For nonresponders, the dosage was titrated up to a maximum of 40 mg. Active treatment continued for 12 weeks. Initial blood pressures (mean +/- SD) were DBP, 102 +/- 6.1 mm Hg, and SBP, 163 +/- 14.4 mm Hg. Final blood pressures were DBP, 83 +/- 6.5 mm Hg, and SBP, 135 +/- 11.6 mm Hg. The response rate for the therapeutic goal (DBP < 90 mm Hg and SBP < 140 mm Hg, or a reduction in SBP > or = 20 mm Hg) was 67.1%; 41 patients did not complete the study. The most common adverse events were cough, headache, and dizziness; only 10 patients (1.3%) failed to complete the study because of adverse events. Quinapril, as used in current private clinical practice, is well tolerated and effective for the treatment of patients with stage 1 or 2 hypertension.