Beta-adrenergic blocking drugs are gaining acceptance as initial therapy for patients with mild to moderate hypertension. In a postmarketing surveillance study, 5,190 hypertensive patients received timolol maleate monotherapy and were evaluated by 1,355 physicians. A total of 1,057 patients did not complete the study: 28% of these patients experienced an adverse event. Mean systolic and diastolic blood pressure readings were reduced 20 and 13 mm Hg, respectively. Mean diastolic blood pressure was reduced 11% for patients with mild hypertension; larger mean reductions were noted for patients with moderate (17%) and severe hypertension (22%). The effect in black and elderly patients was less than in other groups. Although 22% of all patients experienced an adverse event, less than 2.2% of all patients experienced events related to beta-adrenergic blockade, ie, respiratory difficulty, heart failure, bradycardia, and cold extremities. Fatigue, dizziness, and nausea were the most frequently reported adverse events requiring discontinuation of therapy. Timolol monotherapy is a well-tolerated and effective treatment for a broad range of hypertensive patients.