The efficacy of once-daily bopindolol, a nonselective beta-adrenoceptor blocking agent with partial agonist activity, and of pindolol and atenolol in the treatment of essential hypertension has been compared. 369 patients were investigated in a double-blind parallel-group study. The treatment period was 10 weeks. Blood pressure normalisation (diastolic BP equal to or less than 90 mmHg) was to be achieved by a stepwise increase in the dose of the test drugs, and, if required, by addition of a diuretic. Normalisation of blood pressure was achieved in 71 to 76% of the subjects, with no significant differences between bopindolol, pindolol, and atenolol. Special attention was given to evaluation of side effects by using two methods for registration of all adverse events during the study. A low incidence of drug-induced side effects was observed, with no significant difference between bopindolol, pindolol, and atenolol. There was no evidence of unsuspected adverse reactions due to bopindolol.