The antihypertensive efficacy and tolerability of bisoprolol and atenolol were compared in the Bisoprolol International Multicenter Study (BIMS). In 104 patients with essential hypertension (21 to 70 years of age and diastolic blood pressures (DBP) of 100-120 mm Hg in the sitting position), after a four-week placebo period, the active drug was given in a random double-blind crossover design (10 to 20 mg bisoprolol, 50 to 100 mg atenolol) for eight weeks each, with a 2 to 6 week placebo wash-out phase between active therapy. All blood pressures were recorded 24 h after drug intake. Of the 104 patients, 10 were withdrawn during the study because of unwanted effects, unsatisfactory blood pressure response, or intercurrent disease, leaving 94 for intraindividual comparisons. After 8 weeks active treatment with individually titrated dose, there were slightly, but significantly greater falls in blood pressure with bisoprolol (p less than or equal to 0.05). Diastolic target pressures of less than or equal to 95 mm Hg were reached in 68% with bisoprolol and 56% with atenolol (p less than or equal to 0.05; Mc Nemar). There was no between treatment group difference in the patients' self-assessed well-being. For both drugs, response rates were greater in patients below the age of 60 years than in those above 60 years. Better antihypertensive responses were observed with bisoprolol in patients regularly smoking cigarettes.