AIM: To assess all clinically and bacteriologically documented episodes of spontaneous bacterial peritonitis diagnosed in a single unit over a 20 year period, to identify changes in the nature and antibiotic resistance of the causative bacteria. SETTING: A specialist liver disease unit in a tertiary care centre. MATERIAL: Cultured ascitic fluid obtained in the course of 240 consecutive episodes of clinically and bacteriologically proven spontaneous bacterial peritonitis. Patient recruitment remained stable during the 20 year period in terms of the number of cirrhotic patients admitted and the severity of their condition. RESULTS: 78.7% of isolates were Enterobacteriaceae (Escherichia coli in 51%) and 19% were Gram positive cocci. Until 1979 all the Enterobacteriaceae had the wild phenotype, compared with only 50% at the end of the study period. Since 1993, 22% of Enterobacteriaceae have been resistant to third generation cephalosporins. Methicillin resistant staphylococci were only isolated after 1989. CONCLUSIONS: Changes in the epidemiology and antibiotic resistance of bacteria causing spontaneous bacterial peritonitis must be monitored for optimal treatment.