In cases of community-acquired peritonitis, the adequacy of emprical antibiotic treatment has been shown to attenuate mortality and morbidity. The impact of empirical antibiotics on the outcome of postoperative peritonitis has never been evaluated. This study included 100 consecutively studied patients with postoperative peritonitis. The adequacy of emprical treatment was determined by means of culture and susceptibility data obtained at the time of reoperation, and the effect of such treatment on outcome was evaluated. One hundred resistant pathogens were isolated from 70 patients, of whom 45% died; by comparison, mortality among those from whom susceptible organisms were isolated was 16% (P < .05). Inadequate empirical treatment was administered to 54 patients and was associated with poorer outcome (P < or = .05). The outcome of postoperative peritonitis is affected by the choice and adequacy of the initial empirical antibiotic therapy. Late changes in antibiotic therapy based on culture results did not affect outcome when the initial regimen was inadequate.