The objective of this study was to compare the antihypertensive efficacy and tolerability of losartan potassium (losartan) and atenolol in patients with mild-to-moderate essential hypertension. This was a multinational, prospective, randomized, 12-week double-blind parallel study with a follow-up of 4 to 10 days posttreatment to assess any adverse effects of abrupt therapy withdrawal. Two hundred two patients were randomized (2:1) to treatment with losartan or atenolol, 50 mg once daily. Patients were titrated after 6 weeks to 100 mg once daily if their blood pressure was uncontrolled (sitting diastolic blood pressure > or = 90 mm Hg). Trough sitting diastolic blood pressure reductions at weeks 6 and 12 were similar in both the losartan (-9.2 mm Hg and -8.3 mm Hg) and atenolol (-10.8 mm Hg and -10.1 mm Hg) groups and a similar percentage of patients responded to each drug. Both agents were generally well tolerated, although eight patients (two patients taking losartan, and six taking atenolol) were withdrawn because of clinical adverse events (P < or = .05). Reduction in pulse rate from baseline averaged 10 beats/min in the atenolol group with no pulse rate reduction observed in the losartan group (P < .01). No evidence of rebound hypertension was observed in either group. In conclusion, losartan was as efficacious as atenolol in blood pressure reduction, and was at least as well tolerated.