To assess the association between inappropriate antibiotic therapy and clinical outcomes for complicated community-acquired intra-abdominal infections in Spain, patient records from October 1998 to August 2002 in 24 hospitals were reviewed. Initial empiric therapy was classified appropriate if all isolates were sensitive to at least 1 of the antibiotics administered. Multivariate analyses were performed to assess associations between appropriateness of therapy and patient outcomes. Healthcare resource use was measured as hospital length of stay (LOS) and d on intravenous antibiotic therapy. A total of 425 patients were included. Of these, 387 (91%) received appropriate initial empiric therapy. Patients on inappropriate therapy were less likely to have clinical success (79% vs 26%, p<0.001), more likely to require additional antibiotic therapy (40% vs 7%, p<0.01) and more likely to be re-hospitalized within 30 d of discharge (18% vs 3%, p<0.01). Multivariate analyses also showed that inappropriate therapy was associated with an almost 16% increase in LOS (p<0.05) and 26% in d of intravenous antibiotic therapy compared with appropriate therapy (p<0.05). Inappropriate initial antibiotic therapy was associated with a significantly higher proportion of unsuccessful patient outcomes (including death, re-operation, re-hospitalization or additional parental antibiotic therapies), increased length of stay and length on therapy.