Bopindolol is a new beta blocker with a long duration of action which exerts antihypertensive effects at very low dosage. The aim of this study was to define the optimal dosage of bopindolol in outpatients with mild to moderate hypertension and to compare it to atenolol in term of tolerance and efficacy, both administered once daily. Forty physicians (the Swiss Cooperative Study Group) participated as investigators in this study. They had recruited 257 hypertensive patients whose diastolic blood pressure was between 95 and 105 mm Hg (age 19-69 years; mean +/- SD, 49.3/49.9 +/- 10/11) in a multicentric double-blind study. After a 2-week placebo period patients were randomized to one of four treatment groups: bopindolol 1 mg, bopindolol 2 mg, atenolol 50 mg, and atenolol 100 mg. After 4 weeks of active treatment, if diastolic blood pressure was above 90 mm Hg, the dosage was doubled. A detailed evaluation was performed after 8 weeks of active treatment. If necessary, a diuretic was then added and patients were followed for the next 10 months in an open study. Compared to placebo, blood pressure was significantly lowered in the four treatment groups. At 4 weeks, 8 weeks, and 12 months, there was no significant difference between the blood pressures of the treatment groups. Bopindolol was as well tolerated as atenolol. Bradycardia was more common with atenolol. During this 12-month study, bopindolol 1 mg per day appeared to be an effective and a safe antihypertensive treatment. With the exception of the lower incidence of bradycardia, bopindolol is indistinguishable from atenolol in terms of side effects pattern and frequency.