The Medical Research Council's treatment trial for mild hypertension was designed to determine the effects of blood pressure reduction on cardiovascular morbidity and mortality rather than to compare the separate effects of thiazides and beta-adrenergic blocking agents. However, the simultaneous use of both active treatments was discouraged and comparisons in terms of blood pressure control, adverse drug reactions, and drug-related changes in the serum biochemistry are possible. The net differences in systolic and diastolic pressure between treated and control subjects were greater in older than in younger people; this net difference was more pronounced in the older people assigned at random to receive bendrofluazide as opposed to propranolol; this effect increased with time during the trial. The need for a supplementary drug (methyldopa) to control blood pressure at target level, was greater in the thiazide-treated group for all ages. Withdrawal from randomized treatment due to adverse reactions was greater in men receiving bendrofluazide than in those receiving propranolol, and greater for women receiving propranolol than in those receiving bendrofluazide. Thiazide treatment was shown, in a sub-study, to be associated with a significant increase in ventricular ectopic activity in long-term participants.