The rate and extent of return of renal function and its maintenance after initial improvement and factors influencing renal recovery were studied in 186 patients with acute renal failure. Renal function was evaluated by standard clearance techniques (Cinulin, CPAH (para-aminohippurate)) in 40 of 87 survivors. The interval between the episode of acute renal failure and follow-up averaged 45.0 months. Of 36 patients studied more than 3 months after recovery, 22 (61%) had "incomplete" renal functional recovery, or Cinulin and CPAH values more than 1 SD below expected values. Patients with impaired clearances were older at the onset of acute renal failure and had a longer period of oliguria than patients who had "complete" recovery. Failure to regain expected clearance values was apparent in the third decade, and patients 40 years of age or older attained at most 75% of normal values. Serial observations in some suggested a decline in renal function after initial improvement.