BACKGROUND In the critically ill, ischemia secondary to decreased renal blood flow (RBF) is believed to be central to the pathogenesis of acute renal failure (ARF); however, the scientific basis for this conclusion has not been systematically evaluated. METHODS Systematic interrogation of the Pubmed database, and screening bibliographies of retrieved reports, for studies of human ARF where RBF was measured. RESULTS Thirty-two articles published between 1944 and 2008 describing RBF in 373 patients with ARF were identified. Overall, mean RBF during ARF was 387 ml/min. It was 329 ml/min when estimated by clearance-based techniques (15 studies) and 471 ml/min when measured with nonclearance-based techniques (17 studies). Only 46 patients had measurements in the intensive care unit where mean RBF was 306 ml/min. Normal RBF was reported in 14 publications, mean 1,192 ml/min. CONCLUSIONS Limited information is available on RBF during ARF in the critically ill. Measurements in contemporary patients are required to further our understanding of this condition.