In a randomized trial conducted in 35 centers, we compared the clinical efficacy and safety of piperacillin plus tazobactam (TAZ) alone (monotherapy [MT]) versus those of TAZ combined with amikacin (AMK) (combined therapy [CT]) for the treatment of severe generalized peritonitis (SGP). Primary analysis consisted of blind assessment by an independent committee of the failure rate 30 days after the end of treatment in the modified intent-to-treat (ITT) analysis (mITT) population. Of the 241 patients with suspected SGP randomized into the study, 227 were eligible for ITT analysis, including 204 (99 in the MT group and 105 in the CT group) with confirmed SGP (mITT population). A total of 159 patients were eligible for per-protocol (PP) analysis. The clinical failure rates were equivalent in the mITT and PP populations (MT versus CT): 56 versus 52%, (odds ratio [OR] 0.87, 90% confidence interval [CI] = 0. 6 to 1.27) for mITT and 49 versus 49% (OR = 1.03, 90% CI = 0.67 to 1. 59) for PP analysis. Mortality rates (ITT population, 19%; PP population, 21%) and overall adverse event rates (ITT population, 55%; PP population, 54%) were also similar. Six patients (three in MT group and three in the CT group) developed acute renal failure. In conclusion, the addition of AMK to TAZ does not seem to be necessary for the treatment of SGP, even after adjustment for the simplified acute physiology score (SAPS II) and type of SGP.