The antihypertensive and renal effects of the angiotensin-converting enzyme inhibitor lisinopril were studied in a group of patients with moderate-to-severe hypertension and impaired renal function. After 12 weeks of treatment, most patients had good blood pressure response to lisinopril monotherapy. During this period, correlations between antihypertensive effect, drug dose, and serum drug level were observed. These correlations were no longer evident after prolonged treatment. During a 1-year follow-up period, the drug dose was lowered gradually without losing antihypertensive effect. Hyperkalemia occurred in one third of the patients. During the 1-year follow-up, the glomerular filtration rate (GFR) decreased in two thirds of the patients and remained stable in the other third. In this latter group, the pretreatment GFR was higher, and the effective renal plasma flow had increased, whereas in the patients with a decreased GFR no renal vasodilation had occurred during lisinopril therapy. Thus, lisinopril is an effective antihypertensive drug for patients with impaired renal function. The dose should be adjusted to the pretreatment GFR, and a decrease in dosage should be considered with prolonged treatment.