Prescribing patterns and blood pressure control have been studied in 1101 patients treated at three specialist hypertension clinics in Britain. Seventy-four per cent of the patients were already receiving treatment at the time they were referred by their general practitioners. Though the initial improvement in blood pressure control was satisfactory, there was often some deterioration of control over the long term. The pressure exceeded 140/90 mmHg in the majority of patients followed for a year or more. During 1971-5 diuretics remained first preference, with increasing use of beta-blockers and a distinct decline in the use of sympathetic neurone blockers both by hospital staff and referring practitioners. Multiple drug treatment was common, nearly half the patients requiring more than one antihypertensive drug. In newly treated patients the frequency of cessation of a particular class of drug because of side effects or lack of efficacy ranged from 6 per cent with diuretics to 57 per cent with adrenergic neurone blocking drugs. Long-term blood pressure control still presents many problems, and the results contrast with the more optimistic interpretations sometimes placed on short-term clinical trials of antihypertensive effect.