In 15 patients about to undergo colorectal resection for cancer (group I), body weight, lymphocytes, albumin, prealbumin, complement components C3 and C4 and total hemolytic complement activity (TC) were evaluated on admission to hospital and after preroperative preparation which involved dietary restrictions for 5-9 days (450 calories, 25 g protein daily). The same parameters were evaluated in 15 patients admitted for other major surgery not requiring colonic preparation (group II). Statistically significant reduction in prealbumin, C3, C4 and TC were found only in group I. The observations suggest a relationship between immunonutritional status and preparation for large-bowel surgery, reproducing a state of acute malnutrition.