Objective: To study the effects of the initiation time of continuous venovenous hemofiltration and of the ultrafiltrate rate in patients with circulatory and respiratory insufficiency developing early oliguric acute renal failure. The primary end points were mortality at 28 days and recovery of renal function. Design: A randomized, controlled, two-center study. Setting: The closed-format multidisciplinary intensive care units of a university hospital (30 beds) and a teaching hospital (18 beds). Patients and Interventions: A total of 106 ventilated severely ill patients who were oliguric despite massive fluid resuscitation, inotropic support, and high-dose intravenous diuretics were randomized into three groups. Thirty-five patients were treated with early high-volume hemofiltration (72-96 L per 24 hrs), 35 patients with early low-volume hemofiltration (24-36 L per 24 hrs), and 36 patients with late low-volume hemofiltration (24-36 L per 24 hrs). Results: Median ultrafiltrate rate was 48.2 (42.3-58.7) mL·kg-1·hr-1 in early high-volume hemofiltration, 20.1 (17.5-22.0) mL·kg-1·hr-1 in early low-volume hemofiltration, and 19.0 (16.6-21.1) mL·kg-1·hr-1 in late low-volume hemofiltration. Survival at day 28 was 74.3% in early high-volume hemofiltration, 68.8% in early low-volume hemofiltration, and 75.0% in late low-volume hemofiltration (p = .80). On average, hemofiltration started 7 hrs after inclusion in the early groups and 42 hrs after inclusion in the late group. All hospital survivors had recovery of renal function at hospital discharge, except for one patient in the early low-volume hemofiltration group. Median duration of renal failure in hospital survivors was 4.3 (1.4-7.8) days in early high-volume hemofiltration, 3.2 (2.4-5.4) days in early low-volume hemofiltration, and 5.6 (3.1-8.5) days in late low-volume hemofiltration (p = .25). Conclusions: In the present study of critically ill patients with oliguric acute renal failure, survival at 28 days and recovery of renal function were not improved using high ultrafiltrate volumes or early initiation of hemofiltration.