To assess the suitability of therapy with an angiotensin-converting enzyme inhibitor for older hypertensives, a randomised double blind two group comparison of enalapril (10 to 40 mg daily) with hydrochlorothiazide (25 to 100 mg daily) was carried out in 32 untreated and independently living subjects aged 63 to 82 years with pressures on screening greater than 100 mmHg diastolic or greater than 180 mmHg systolic and after two weeks on placebo greater than 90 mmHg diastolic or greater than 160 mmHg systolic. Twenty-five subjects completed the ten weeks of active treatment with the majority in both groups reaching target blood pressures. Four subjects on enalapril and three on hydrochlorothiazide withdrew because of side effects. Eighteen of the 32 patients volunteered troublesome symptoms they attributed to therapy, which in some instances were alleviated by reductions in drug dosage. Small but significant falls in plasma potassium and sodium and increases in plasma urea and uric acid were seen on hydrochlorothiazide only. It was concluded that although in most cases good blood pressure control can be achieved by enalapril with some advantages over other drugs in the elderly, the potential for adverse effects remains significant. Halving usual starting doses of this and other antihypertensives may help to minimise problems in older subjects.