In a comparative clinical trial to examine the influence of 10 days of preoperative parenteral nutrition (PPN) on the postoperative complication rate for gastrointestinal carcinoma 59 patients (controls) received the regular hospital diet and 66 received PPN. The two groups were similar in nutritional status and in distribution of site and stage of tumour and type of operation. The rates of postoperative wound infection, pneumonia, major complications, and mortality were generally lower in the PPN group, but the differences were significant only for major complications and mortality. The clinical results can be explained by the improvement in various indices of humoral and cellular immunocompetence and the protein status in the PPN group and their deterioration in the control group during the preoperative course.