To delineate the clinical spectrum of nonliguric renal failure, we studied prospectively 90 patients with acute renal failure 54 of whom were nonoliguric throughout their periods of azotemia. Although the causes of nonoliguric renal failure varied, nephrotoxic failure occurred more frequently in nonoliguric than in oliguric subjects (P is less than 0.01). As com pared to oliguric patients, those without oliguria had significantly lower urinary sodium concentrations (P is less than 0.05) and fractional excretions of sodium (P is less than 0.02), had shorter hospital stay (P is less than 0.01), had fewer septic episodes, neurologic abnormalities, gastrointestinal bleeding and acidemia, required dialysis less frequently (P is less than 0.001) and had lower mortality rate (26 per cent in nonoliguric vs. 50 per cent in oliguric patients -- P is less than 0.05). Nonoliguric renal failure occurs more often than is generally recognized and causes less morbidity and mortality than oliguric acute renal failure.