The effects of cilazapril 2.5 to 5.0 mg every 24 hours orally for one month on blood pressure, renal hemodynamics, and plasma and urinary hormones were investigated in 11 patients with renal disease and hypertension. Six patients had renal failure (pretreatment creatinine clearance of less than 1 ml/second). Both systolic and diastolic blood pressures were significantly decreased in all patients and seven of the 11 required 5.0 mg daily. There was no significant change in the effective renal plasma flow, glomerular filtration rate, or creatinine clearance, although the renal vascular resistance was significantly reduced. Albuminuria was reduced in most patients but the mean change was not significant. The mean plasma potassium concentration rose significantly, but no changes in electrolyte excretion or weight were noted. Mean ambulant plasma renin activity was normal before treatment and rose significantly. Mean ambulant angiotensin II concentrations did not change. Mean ambulant plasma aldosterone concentration and mean urinary aldosterone excretion decreased significantly. Plasma arginine vasopressin and cortisol concentrations did not change. No changes were noted in plasma glucose concentrations, liver function tests, hemoglobin concentrations, or white blood cell or platelet counts. A slight looseness of bowel motions occurred in two patients and was the only side effect recorded. Cilazapril appears to be an effective and safe antihypertensive drug in patients with hypertension and renal disease.