OBJECTIVE: A double-blind, randomized, multicenter clinical study comparing ciprofloxacin/metronidazole (CIP/MET) versus piperacillin/tazobactam (PIP/TAZO) in hospitalized adult patients (pts) with severe intraabdominal infections was the basis for this pharmacoeconomic analysis. METHODS: A cost-effectiveness analysis from a hospital provider perspective was conducted. Cost-effectiveness ratios (CER) were calculated RESULTS: Among 276 evaluable pts, 148 received CIP/MET and 128 received PIP/TAZO. IV-PO: 65% (96/148) CIP/MET and 59% (75/128) PIP/TAZO pts were switched to oral administration. IV-PO CIP/MET pts with appendicitis had an 89% success rate; mean treatment duration was 6.4 ± 3.4 days and mean CER was US$7,696. Similar pts who received PIP/TAZO had a 68% success rate; mean treatment duration was 5.4 ± 2.7 days and mean CER was US$10,372. IV-PO CIP/MET non-appendicitis pts had an 83% success rate; mean treatment duration was 7.7 ± 2.6 days and mean CER was US$11,114. Similar pts who received PIP/TAZO had a 72% success rate; mean treatment duration was 8.5 ± 4.8 days and mean CER was US$13,331. .IV Only: CIP/MET appendicitis pts had a success rate of 87%; mean therapy duration was 5.0 ± 2.2 days and mean CER was US$7,328. Similar pts who received PIP/TAZO had a 56% success rate; mean treatment duration was 6.4 ± 3.1 days and mean CER was US$10,980. Non-appendicitis CIP/MET pts had a success rate of 45%; mean therapy duration was 13.3 ± 9.5 days and mean CER was US$17,881. Similar pts who received PIP/TAZO had a 52% success rate; mean treatment duration was 11.6 ± 8.2 days and mean CER was US$17,944. Sensitivity analysis revealed that PIP/TAZO needed to be 10-15% more successful in order to be cost-effective. CONCLUSION: Whether pts had appendicitis or not, and whether they could be switched to oral antibiotics or not, the combination of ciprofloxacin/metronidazole was found to be cost-effective compared to piperacillin/tazobactam.