In the control of chronic disease no therapeutic regimen is successful unless it is complied with. A number of studies have indicated that compliance with tablet-taking may be as low as 40%. Patients with hypertension are frequently on a number of different anti-hypertensive agents, and if they have other chronic disorders they may take as many as 10 different drugs and up to 40 tablets per day. It is therefore not surprising that compliance is poor. To achieve compliance requires education of the patient, reduction in the number of drugs and simplification of the drug regimen. Methyldopa was used in a crossover study on a once- or twice-daily basis. Blood pressure was measured at the same time each day 2 hours after the morning dose. Compliance was assessed by tablet count and by blood pressure control, which was better on once-a-day therapy. Over a 6-week period 95% of medication was taken on the once-daily compared with 84% on the twice-daily regimen. In a subsequent study atenolol once per day replaced propranolol given 3 times per day. Blood pressure was lower on atenolol and tablet compliance was 94% compared with 74% on thrice-daily propranolol therapy. In addition, many patients admitted not taking the midday dose. The effect of dietary advice was then monitored by 24-hour urine electrolytes. When advice was given superficially by the doctor, urine sodium fell from 186 mmol/day to 165 mmol/day. When seen on one occasion by a dietitian and given diet sheets, it fell from 182 to 135 mmol/day. When seen at repeated visits by the dietitian and the advice modified according to sodium excretion, urine sodium excretion fell from 188 to 83 mmol/day. Supplemental oral potassium is often given as antihypertensive medication and up to 6 tablets per day may be administered. Compliance decreased as the number of tablets increased. Compliance was 92% on 1 tablet, 83% on 2 tablets, 68% on 3 tablets, 75% on 4 tablets (usually taken as 2 tablets twice a day) and 58% when on 6 tablets per day. The compliance with diuretic-taking was 96%. When given amiloride/hydrochlorothiazide the compliance was 93% and this elevated plasma potassium more than high dose supplemental potassium. In a recent study people on 3 or more drugs for blood pressure control were placed on a low salt diet and their drugs replaced with enalapril.(ABSTRACT TRUNCATED AT 400 WORDS)