To evaluate the influence of renal function on the efficacy of converting-enzyme inhibition in patients with severe chronic heart failure, we measured the long-term hemodynamic and clinical responses to captopril in 101 consecutive patients with heart failure grouped according to pretreatment serum creatinine concentration (group I, less than 1.4 mg/dL; group II, 1.4 to 2.8 mg/dL; and group III, greater than 2.8 mg/dL). After 1 to 3 months of treatment, patients with preserved renal function (group I) had greater increases in stroke volume index and greater decreases in left ventricular filling pressure and systemic vascular resistance than did patients with renal impairment (groups II and III) (p less than 0.05). Clinical improvement paralleled these hemodynamic benefits; only 2 of 12 patients with severe renal insufficiency (group III) improved clinically compared with 29 of 40 patients with preserved renal function (group I) and 29 of 49 patients with mild-to-moderate renal impairment (group II), (p less than 0.005). Therapy-limiting rash and dysgeusia occurred most frequently in patients with renal impairment. Our findings support an important role for the kidneys in mediating the beneficial actions of captopril in patients with severe congestive heart failure.