Background: This study was performed to investigate the risk factors for a first episode of spontaneous bacterial peritonitis in cirrhotic patients. Methods: One hundred ten cirrhotics with sterile ascites, without previous spontaneous bacterial peritonitis (SBP), were included from March 1988 to October 1989 and followed up until October 1990 (follow-up, 46 ± 3.5 weeks; range, 4-120 weeks). Results: Twenty-eight patients (25.45%) suffered SBP. In multivariate analysis (Cox's regression model) including only variables commonly used in clinical practice, ascitic fluid protein concentration and serum bilirubin level independently correlated with first SBP development. Using these two variables the relative risk of a first SBP episode was calculated for each patient. According to the median relative risk coefficient (1.2), a low-risk group (relative risk, <1.2) and a high-risk group (relative risk, >1.2) were established. Kaplan-Meier estimates of patients free of SBP were significantly higher in the low-risk group. Conclusions: The probability of a first SBP episode is significantly influenced by the antimicrobial capacity of ascitic fluid and hepatic function. © 1993.