Five hundred patients with acute renal failure were admitted to the Department of Nephrology of Necker Hospital between 1966 and 1970; 279 of them were treated before and 221 after beginning systematic prophylactic hemodialysis. The frequency of complications and the survival rate between both groups were compared. No significant improvement was obtained in patients with severe pre-existing diseases or extensive extrarenal lesions; such patients usually died from nonuremic complications before, or despite, dialysis. However, adequate treatment of severe septic shock, when present, was an important factor in improving the immediate outcome. Early and frequent dialysis significantly reduced the frequency and intensity of uremic symptoms such as gastrointestinal hemorrhage. The mortality due to gastrointestinal bleeding decreased from 14% before to 5% after prophylactic hemodialysis. The number of deaths due to septicemia was also substantially lowered from 24 to 12%. Overall mortality was reduced from 42 to 29%. The mortality rate decreased significantly in all categories of patients, particularly in surgical cases (from 54 to 38%) and in traumatic cases (55 to 33%). Simultaneously, the better use of antibiotic therapy, better prophylaxis of sepsis, and better nutritional care were important factors in the overall improvement in prognosis.