We undertook a study to determine the frequency, predisposing factors, and outcome in 315 patients admitted to a medical-surgical ICU, of whom 47 (14.9%) subsequently acquired renal insufficiency (ARI) during their stay in the unit. Four well-recognized risk factors for ARI were present alone or in combination in all episodes: hypotension, sepsis, aminoglycoside antibiotics, and radiocontrast dye. Hypotension was the most prevalent factor, present in 42 (85.8%) episodes, and was the sole factor present in 18 (36.7%). Patients with ARI but without hypotension all survived their ICU stay, while only 13 (33%) of 40 with hypotension survived (p less than .05). Neither initial, peak nor change in BUN or creatinine predicted mortality; oliguria was marginally associated with poor prognosis. Our findings indicate that: a) ARI was a frequent and important contributing factor to mortality in our critically ill patients, b) hypotension was the most common of well-recognized risk factors, and c) conditions that predisposed to ARI also predisposed to mortality, although mortality did not appear to depend on the severity of renal insufficiency.