Introduction Despite significant improvements in intensive care medicine, the prognosis of acute renal failure (ARF) remains poor, with mortality ranging from 40% to 65%. The aim of the present observational study was to analyze the influence of patient characteristics and fluid balance on the outcome of ARF in intensive care unit (ICU) patients. Methods The data were extracted from the Sepsis Occurrence in Acutely Ill Patients (SOAP) study, a multicenter observational cohort study to which 198 ICUs from 24 European countries contributed. All adult patients admitted to a participating ICU between 1 and 15 May 2002, except those admitted for uncomplicated postoperative surveillance, were eligible for the study. For the purposes of this substudy, patients were divided into two groups according to whether they had ARF. The groups were compared with respect to patient characteristics, fluid balance, and outcome. Results Of the 3,147 patients included in the SOAP study, 1,120 (36%) had ARF at some point during their ICU stay. Sixty-day mortality rates were 36% in patients with ARF and 16% in patients without ARF (P < 0.01). Oliguric patients and patients treated with renal replacement therapy (RRT) had higher 60-day mortality rates than patients without oliguria or the need for RRT (41% versus 33% and 52% versus 32%, respectively; P < 0.01). Independent risk factors for 60-day mortality in the patients with ARF were age, Simplified Acute Physiology Score II (SAPS II), heart failure, liver cirrhosis, medical admission, mean fluid balance, and need for mechanical ventilation. Among patients treated with RRT, length of stay and mortality were lower when RRT was started early in the course of the ICU stay. Conclusion In this large European multicenter study, a positive fluid balance was an important factor associated with increased 60-day mortality. Outcome among patients treated with RRT was better when RRT was started early in the course of the ICU stay.