Spontaneous bacterial peritonitis is diagnosed when (a) the ascitic fluid culture is positive, (b) the ascitic fluid neutrophil count is greater than or equal to 250 cells/mm3 and (c) there is no evident intraabdominal surgically treatable source for infection. Few details are available regarding the variant of ascitic fluid infection in which the culture grows bacteria (pure growth of a single type of organism), but the neutrophil count is less than 250 cells/mm3. In this prospective study of 138 episodes of culture-positive spontaneously infected ascites detected in 105 patients, 44 (31.9%) were episodes of "monomicrobial nonneutrocytic bacterascites" compared with 94 (68.1%) episodes of spontaneous bacterial peritonitis. Seventeen patients had both types of infection. The infection-related mortality and hospitalization mortality were similar between the two groups. Patients with bacterascites appeared to have less severe liver disease. In 62% of bacterascites episodes in which a second paracentesis was performed before any treatment the fluid spontaneously became sterile without development of ascitic fluid neutrocytosis. Thirty-eight percent of patients with bacterascites (who underwent a second paracentesis before treatment was started) progressed to spontaneous bacterial peritonitis--sometimes within a few hours. The concentration of the chemoattractant C5a was not decreased in the ascitic fluid of the bacterascites patients; this excludes ascitic fluid C5a deficiency as the explanation of the lack of neutrocytosis. Monomicrobial nonneutrocytic bacterascites is a common variant of ascitic fluid infection that may resolve without treatment or may progress to spontaneous bacterial peritonitis.