The diuretic effect of high doses of furosemide alone and furosemide plus maniitol was analysed retrospectively in 30 children with acute renal failure. In 10 children (Group 1) renal failure developed mainly during glomerulonephritis, and in 20 children (Group 2) the cause was gastroenteritis. The diuretic effects of furosemide and furosemide plus mannitol were evaluated measuring the 24-hour urine volume at the time of anuria, oliguria or normal diuresis. The highest mean single intravenous doses of furosemide were 6.5 and 14 mg/kg in Groups 1 and 2, respectively; the highest average daily doses were 10.1 and 25.5 mg/kg, respectively. A broad relationship was observed between single i.v. dose and diuretic response following administration of furosemide (1.2 to 30.8 mg/kg). In both groups of patients a statistically significant negative linear correlation was found between the daily intravenous dose of furosemide and the 24-hour urine volume. Calculations based on the obtained regression equations showed that the expected 24-hour urine volumes corresponding to daily diuresis normal for age could be obtained after administration of daily 2.8 to 1.4 mg/kg furosemide in Group 1 and 9.3 to 2.3 in Group 2. It is therefore suggested that the total daily dose of furosemide should not exceed 100 mg in children with acute renal failure. Administration of furosemide plus mannitol did not result in higher daily diuresis as compared to 24-hour urine volume obtained when furosemide was given alone. Furosemide was well tolerated. Electrolyte disturbances, especially in Group 2, were the most frequent side effects due to high doses of furosemide.