AIMS: The objective of this population-based, retrospective cohort study was to investigate the incidence and initial antibiotic treatment of secondary intra-abdominal infections (sIAI) and to assess whether inappropriate initial antibiotic therapyaffects patient outcomes. METHODS: All patients hospitalized for sIAI (1995-1998) were identified in the PHARMO Record Linkage System, a patient-centric database including pharmacy dispensing records from community pharmacies linked to hospitalization records in the Netherlands. Complementary in-hospital antibiotic drug use was obtained from the computerized inpatient pharmacy files. The patient outcomes considered were switch to second-line antibiotic treatment, re-operation, and death. In addition, acomposite variable clinical failure was constructed based on the above-mentioned outcomes. Furthermore, the effect of clinical failure on length of hospital stay and costs of hospitalization was assessed. Associations between appropriateness of initial antibiotic treatment and outcomes were estimated using multivariate logistic and linear regression models. RESULTS: In the source population of 228,000 persons, 175 cases were classified as sIAI (mean age 49.3 +/- 24.5, 50.9% male) resulting in an incidence of 2.3/10,000 person-years [95% confidence interval (CI) 2.0, 2.7]. Initial antibiotic treatment was appropriate for 84% of the cases. The risk of clinical failure was 17.1%. Inappropriate initial antibiotic treatment increased the risk of clinical failure 3.4-fold (95% CI 1.3, 9.1). Length of hospital stay and costs of hospitalization were significantly increased for patients with clinical failure. CONCLUSIONS: Inappropriate choice of initial antibiotic therapy in sIAI patients leads to more clinical failure resulting in a longer hospital stay and higher costs of hospitalization compared with appropriate initial antibiotic therapy.