The possible influence on blood polymorphonuclear (PMN) granulocyte functions of the small intestinal shunt operation for obesity was studied in 10 massively overweight patients. They were investigated prior to operation and for 9 months afterwards, when they had lost an average of 32 kg body weight. Preoperatively they showed reduced PMN bactericidal capacity and increased PMN adherence compared with controls of normal weight. During the first 2--4 months postoperatively all patients displayed a gradually increasing bactericidal capacity, which then reached levels similar to the controls and remained so for the rest of the follow-up period. This enhancement was more easily assessed by a new in vitro assay in which each PMN was provided with 30--40 bacteria, than by a standard assay using 2--4 bacteria per granulocyte. PMN adherence decreased during the first postoperative months and then returned to preoperative levels. The changes in PMN functions were not statistically related either to each other or to the continuous loss of body wieght. Thus, impairment of PMN killing function occurring in extremely obese patients became normalized after small bowel shunt operation, while the high adherence remained unchanged.