We report a case of spontaneous bacterial peritonitis caused by Klebsiella pneumoniae in a 34-year-old male recipient shortly after kidney transplantation. On posttransplant day 10, the patient started complaining of severe abdominal pain and nausea. Body temperature was 38.4 degrees C. The abdomen was diffusely tender with rigidity and rebound. Laboratory data showed a normal erythrocyte sedimentation rate and serum creatinine level but a slightly elevated C-reactive protein concentration and leukocytosis of 36,200 cells/mm(3) with 88% neutrophils. Explorative laparotomy revealed diffuse purulent peritonitis without an intraabdominal source of infection, such as intestinal perforation. The peritoneal fluid revealed greater than 1000/mm(3) white blood cells and many gram-negative bacilli. Fluid cultures yielded growth of Klebsiella pneumoniae. The patient responded to antibiotic therapy; he was discharged in good condition. This case report draws attention to the impaired host defense that may predispose to spontaneous bacterial peritonitis in renal transplant recipients and alerts the clinician to the possibility of this rare disease.