BACKGROUND A series of 459 hospitalized adults with complicated intra-abdominal infections participated in a randomized, double-blind, multicenter clinical trial. The present study was conducted to add a pharmacoeconomic analysis to the results. METHODS A cost-effectiveness analysis from the perspective of the hospital provider was carried out. Decision analysis was used to illustrate outcomes and to provide a basis on which to conduct a sensitivity analysis. Cost-effectiveness ratios, representing the cost per expected successfully treated patient, were calculated to determine the most cost-effective alternative. RESULTS Among 244 economically evaluable patients, enrolled from 34 centers in the U.S. and Canada, 131 patients received ciprofloxacin-metronidazole (75% clinical success rate), and 113 received piperacillin-tazobactam (65% clinical success rate; p = 0.06). Switch to oral antibiotics was possible for 81 patients who received ciprofloxacin-metronidazole (85% clinical success rate) and 67 piperacillin-tazobactam patients (70% clinical success rate; p = 0.027). The mean hospital cost was US$10,662 +/- 7,793 for patients in the ciprofloxacin-metronidazole group and $10,009 +/- 7,023 for patients in the piperacillin-tazobactam group (p = 0.492). Significantly lower costs were documented for patients who could be switched to oral antibiotics than for those continued on intravenous antibiotic orders ($8,684 +/- 4,120 vs. $12,945 +/- 10,204, respectively; p < 0.001). Patients with appendicitis had lower mean hospital costs than those with other infections ($7,169 +/- 3,705 vs. $12,097 +/- 8,342, respectively; p < 0.001). The cost-effectiveness ratios were $14,216:1 for patients in the ciprofloxacin-metronidazole group and $15,398:1 for patients in the piperacillin-tazobactam group. CONCLUSIONS The mean hospital costs associated with ciprofloxacin-metronidazole were similar to those of piperacillin-tazobactam for the treatment of adults with complicated intra-abdominal infections. Lower costs were documented for patients able to be switched to oral antibiotics and for patients with appendicitis.