To evaluate the renal protection of enalapril a study was made in 37 patients with chronic renal failure and hypertension. Sixteen patients had diabetic nephropathy with the serum creatinine ranging from 2.0 to 4.0 mg/dl. Twenty-one patients had non-diabetic chronic renal failure with serum creatinine from 2.2 to 6.3 mg/dl. Of 16 patients with diabetic nephropathy, 6 served as control and 10 received enalapril. Nine patients in the non-diabetic chronic renal failure group served as controls, while 12 were given enalapril. The control patients received alpha methyldopa (500 mg/day) for blood pressure control. In the studied patients enalapril was given at the dose of 5-10 mg/day. Over a period of 2 yrs, enalapril attenuated progression of renal failure in patients with diabetic nephropathy at the serum creatinine level from 2 to 2.9 mg/dl (creatinine clearance 21.5-38.4 ml/min) when compared with control patients. At the serum creatinine of 3 to 4 mg/dl progression of renal failure did not differ from control patients. In non-diabetic renal failure progression of renal failure was delayed in patients with serum creatinine level ranging from 2.2 to 5 mg/dl (creatinine clearance 18-42 ml/min); patients with serum creatinine level ranging from 5.5 to 6.3 mg/dl had deterioration of renal function as control patients. Proteinuria was decreased in all patients on enalapril. Thus, for preventing progression of renal failure, enalapril should be given in chronic renal failure with milder degree of renal function impairment; in diabetic nephropathy it should be started earlier at the lower serum creatinine level than in non-diabetic chronic renal failure, yet with comparable creatinine clearance.