Losartan is a novel orally active nonpeptidal antihypertensive agent that specifically blocks the angiotensin II type 1 receptor. This paper compares the short- and long-term safety and tolerability of losartan with those of placebo. Approximately 3800 patients with mild-to-severe essential hypertension were enrolled in 16 double-masked and 4 open clinical trials worldwide. Of these, approximately 2900 were treated with losartan either alone or in combination with other antihypertensive drugs. These trials included patients with diabetes mellitus (n = 133). An additional 5 trials enrolled hypertensive patients with compromised renal function (n = 115) or heart failure (n = 220). Losartan dosages primarily ranged from 10 to 150 mg once daily, with most patients receiving 50 to 100 mg per day. Hypertension trials generally lasted 12 weeks. The most frequently reported adverse events were headache, upper respiratory tract infection, dizziness, and asthenia/fatigue, but only dizziness occurred more frequently (> or = 1%) in the losartan-treated groups. Cough occurred in 3.1% of patients treated with losartan and 2.6% of patients treated with placebo. The overall incidence of clinical and laboratory adverse events in the losartan- and placebo-treated groups was similar among patients with hypertension and either diabetes mellitus, renal impairment, or heart failure. The data suggest that losartan can be safely administered in hypertensive patients with concomitant illnesses. It can be considered for first-line therapy and is suitable as an alternative therapy in patients already experiencing side effects with other agents.