A postmarketing surveillance study in 2273 Canadian office practices provided the largest body of clinical experience to date with the angiotensin-converting enzyme (ACE) inhibitor lisinopril in the treatment of mild to moderate essential hypertension. The principal emphasis in this uncontrolled study was safety, assessed in 10,289 patients. Patients with a diastolic blood pressure > 90 mmHg were considered for the study. Both previously untreated patients and those who were experiencing adverse effects from their current antihypertensive regimen were included. Lisinopril was begun at a dose of 10 mg/day. Subsequent dose adjustments, to a maximum of 40 mg/day, were made to achieve optimal blood pressure control (diastolic blood pressure < or = 90 mmHg or > or = 10 mmHg below baseline for > or = 4 weeks at the same dose). Therapy was continued for a minimum of 4 weeks to a maximum of 12 weeks, with patients examined every 2 weeks. The frequencies of adverse effects and laboratory abnormalities were analyzed in all treated patients. All 10,289 patients enrolled were considered in the analysis of safety. One or more adverse effects were reported for 1593 (15.5%) patients, and 802 (7.8%) withdrew from the study because of adverse effects. The most frequent adverse effects were cough (4.0%), dizziness (2.3%), headache (2.1%), asthenia (1.7%), and nausea (1.0%). The physicians' global assessment rated overall tolerability as very good or good for 77.1% of the patients. Antihypertensive effect was evaluated in 5886 patients who met the criteria for efficacy analysis. The criterion response was attained in 5141 (87.3%) patients, with 68.6% responding to 10 mg/day of lisinopril, 26.3% to 20 mg/day, and 3.2% to 40 mg/day (the other 1.9% responded at nonstandard doses). Lisinopril was safe and well-tolerated. Except for cough, class effects of ACE inhibitors were rarely encountered. The results of the efficacy analysis confirm the established efficacy of lisinopril in patients with mild to moderate essential hypertension.