Although spontaneous bacterial peritonitis is considered a precipitating factor of renal impairment in cirrhosis, no study specifically addressing this problem has been reported. This study was aimed at assessing the incidence, clinical course, predictive factors and prognosis of renal impairment in cirrhotic patients with peritonitis. Therefore, 252 consecutive episodes of spontaneous bacterial peritonitis in 197 patients were analyzed. Clinical and laboratory data obtained before and after diagnosis of peritonitis were considered as possible predictors of renal impairment and hospital mortality. Renal impairment occurred in 83 (33%) episodes, and in every instance it fulfilled the criteria of functional kidney failure. Renal impairment was progressive in 35 episodes, steady in 27 and transient in 21. Blood urea nitrogen and serum sodium concentration before peritonitis and band neutrophils count in blood at diagnosis were independent predictors for the development of renal impairment. Renal impairment was the strongest independent predictor of mortality during hospitalization. Other independent prognostic factors were blood urea nitrogen level before peritonitis, age, positive ascitic fluid culture and serum bilirubin level during infection. These results indicate that renal impairment is a frequent event in cirrhotic patients with spontaneous bacterial peritonitis that occurs mainly in patients with kidney failure before infection. Renal impairment is the most important predictor of hospital mortality in cirrhotic patients with spontaneous bacterial peritonitis. (Hepatology 1994;20:1495–1501).