The ability of perioperative short-term antibiotic prophylaxis to reduce the predictive significance of nutritional indicators for postoperative infections was evaluated in 162 patients undergoing major surgery for gastric or colorectal cancer. All patients were randomly assigned to a group receiving such prophylaxis or a group with postoperative antibiotic treatment. Preoperative serum albumin, total iron-binding capacity and weight loss were the nutritional indicators, and the evaluation included delayed hypersensitivity response. Postoperative infections occurred in 29% of the total series, with highest incidence in the group with postoperative antibiotics (p less than 0.001) and in anergic patients (p less than 0.05). Increased risk of postoperative infection was related also to the number of altered nutritional indicators (p less than 0.005). Multiple logistic analyses showed that the short-term prophylaxis independently contributed to fall in the infection rate and reduced the prognostic importance of nutritional and immunologic factors. Indeed, heightened incidence of postoperative infection was found only when all three nutritional factors were altered.