Twenty-four patients with classical angina of effort completed this double-blind cross-over clinical trial of acebutolol and propranolol. Both acebutolol (100-400 mg) and propranolol (40-160 mg) three times a day produced a significant reduction in the incidence of anginal attacks, consumption of nitroglycerine tablets and increased exercise tolerance. Both drugs produced mild and tolerable side effects which did not require any reduction of doses of the drugs being used. No change in laboratory parameters assessing the function of the liver, the kidneys and the hemopoietic system was found. The commonly accepted dose ratio of 1:2.5 propranolol and acebutolol, respectively, was found to be true in less than 50% of the patients. Therefore, for optimal effect, the dose of acebutolol should be individualized depending upon the response.