The cytotoxic capacities of T lymphocytes and natural killer (NK) cells and the phagocytic functions of polymorphonuclear leukocytes were determined in a group of 20 patients with end stage disease who were being maintained on hemodialysis. The mononuclear cells from these patients were able to mediate normal T-cytotoxic functions in a xenogeneic cell-mediated lympholysis assay and normal NK cytotoxic capacity when tested against K562 target cells. Contrarily, polymorphonuclear functions in these patients were severely impaired. The reduction of Nitroblue tetrazolium in dark formazan (NBT test) and the direct phagocytosis of 2 pyogenic strains (Staphylococcus aureus and Salmonella typhimurium) were equally and deeply depressed in almost all patients. This defect was not restored either by 4 hr of hemodialysis nor by 12 months of treatment. This study confirms a major role for a defect of polymorphonuclear cell functions in the depressed defence of patients on chronic hemodialysis against infections.