The prevalence and prognostic significance of spontaneous bacterial peritonitis were prospectively studied in a series of 82 acute hepatitis patients decompensated with ascites. The in-hospital prevalence of spontaneous bacterial peritonitis was 31.7% (26 of 82 patients). Twenty cases were culture positive, including one with multiple isolates, and six cases were culture negative. E. coli and Klebsiella pneumoniae were the most common pathogens, accounting for 71.4% (15 of 21) of the total isolates, whereas only 9.5% were gram-positive organisms. No significant difference in the age, sex, cause of acute hepatitis, liver biochemistry, prothrombin time and ascites fluid concentration of total protein was noted between patients with spontaneous bacterial peritonitis and those without spontaneous bacterial peritonitis, except that bacteremia was recognized significantly more frequently in the former (57.7% or 15 of 26 patients) than in the latter (25.0% or 14 of 56 patients, p less than 0.005). In addition, patients with spontaneous bacterial peritonitis, when compared with those without spontaneous bacterial peritonitis, were more likely to have kidney failure (57.7% vs. 30.4%, p less than 0.05) and had a significantly higher mortality rate (73.1% vs. 39.3%, p less than 0.01). Among patients without spontaneous bacterial peritonitis, the prevalence of kidney failure and gastrointestinal hemorrhage and the mortality rate in patients with bacteremia (57.1%, 64.3% and 71.4%, respectively) were significantly higher than in those without bacteremia (21.4%, 19.0% and 28.6%, respectively; p less than 0.05, p less than 0.01 and p less than 0.01, respectively). In conclusion, 31.7% of severe acute hepatitis patients with ascites were recognized as having spontaneous bacterial peritonitis; the other 17.1% had bacteremia.(ABSTRACT TRUNCATED AT 250 WORDS)