OBJECTIVES: To assess the association between hospital costs and the appropriateness of empirical antibiotic treatment for community-acquired intra-abdominal infection. METHODS: Patients were identified from hospital discharges from three hospitals for 1993 to 1997. Medical records were obtained to validate the diagnosis and obtain details of antibiotic therapy and its outcome. Valid cases had macroscopic evidence of intra-abdominal infection at operation, therefore all cases had surgical control of infection in addition to antimicrobial therapy. Appropriateness of empirical therapy was assessed from the results of in-vitro sensitivity tests (culture positive cases) and compliance with local antibiotic policies (culture negative cases). RESULTS: We identified 294 valid cases of intra-abdominal infection of whom 162 (55%) were culture positive. Appropriate antibiotic treatment was associated with significantly shorter length of stay and lower investigation costs for patients with positive cultures but not for patients with negative cultures or no test/result (data not shown). Culture +ve cases, appropriate (n=129) vs inappropriate (n=33): mean length of stay 12vs22 days, p=0.0007 boot strap t test, mean investigation cost £250vs£409 p=0.04, mean antibiotic cost £138 vs £128 p=0.9. A log transformed linear regression analysis was carried out on the cost data for the culture positive cases adjusting for five independent variables. Inappropriate antibiotics, increasing age and a higher number of comorbidities and previous admissions were significantly associated with increased cost of hospital stay but not gender. This model accounted for 34% of the variance in cost of hospital stay. For the cost of investigations inappropriate treatment, increasing age and number of comorbidities were all significant predictors accounting for 26% of variance in costs. Number of comorbidities was the only variable significantly associated with the cost of antibiotics. CONCLUSIONS: Appropriate empirical antibiotic treatment of patients with culture positive intra-abdominal infection is strongly associated with length of stay and hospital costs.