Costs involved in using piperacillin 4 g/tazobactam 500 mg, given as intermittent intravenous infusions every 8 hours, were compared with those for imipenem/cilastatin 500 mg, given as intermittent intravenous infusions every 6 hours, for the treatment of patients with gangrenous or perforated appendicitis. A total of 88 patients were included in our cost analyses: 42 patients in the piperacillin/tazobactam group and 46 patients in the imipenem/cilastatin group. Durations (mean +/- SD) of antibiotic therapies were 7.8 +/- 3.3 days and 7.1 +/- 2.6 days for the piperacillin/tazobactam and imipenem/cilastatin groups, respectively. No statistical significance was found for the difference in duration of therapy (P = 0.376). Total drug treatment costs were $538.83 +/- $385.33 for the piperacillin/tazobactam group and $687.66 +/- $345.37 for the imipenem/cilastatin group. This difference in treatment cost was statistically significant (P = 0.0001). The need for laboratory tests and the use of other medications were not different between the two groups. Total hospital-days charges were higher for the piperacillin/tazobactam group ($18,339.76 +/- $6090.38) compared with the imipenem/cilastatin group ($16,150.00 +/- $5088.60) (P = 0.052). These findings suggest that length of hospital stay should be the economic focus of antibiotic therapy.