Although it has been well documented that chronic hemodialysis (HD) patients are highly susceptible to infectious diseases, the reasons for this have yet to be clarified. The present study was thus designed in order to better define this issue. Fifty-eight stable chronic HD patients without any evidence of infection were selected for the study. Blood samples were collected before and after HD from the same patient to determine the effect of HD. Reactive oxygen species (ROS) production in polymorphonuclear cells (PMNC) was measured by chemiluminescence using luminol. When the PMNC collected after HD were stimulated in vitro with a calcium ionophore (A23187), they produced a larger amount of ROS than that obtained from healthy volunteers [mean 7.4 × 10<sup>5</sup> photon counts (n = 58) vs. 3.0 × 10<sup>5</sup> photon counts (n = 17); p < 0.01]. A higher production of ROS after HD was seen in patients using membranes such as cellulose triacetate, polymethylmetacrylate and cellulose diacetate, whereas cuprophane did not seem to augment ROS production at all. On the other hand, when the PMNC after HD were stimulated with phorbol myristate acetate, their photon counts (mean 4.3 × 10<sup>7</sup>) were comparable to those before HD (mean 3.5 × 10<sup>7</sup>), and to those of PBMC obtained from healthy volunteers (mean 4.1 × 10<sup>7</sup>). It was thus suggested that the enhanced ROS production of PMNC was related to some stimuli, possibly even to the assay used to measure ROS. The phagocytotic activity and bactericidal effect ofPBMC were measured by coculturing 1 × 10<sup>5</sup> PMNC with 1 × 10<sup>5</sup> CFU of Escherichia coli. Similar phagocytotic activities were noted in the PMNC from healthy volunteers and chronic HD patients before and after HD: the mean number of phagocytosed bacteria (log<sub>10</sub> CFU) was 3.3, 3.3, and 3.3, respectively. However, in the case of a bactericidal effect, only the PMNC from healthy volunteers, but not the PMNC from HD patients, could effectively kill the bacteria, since the number of bacteria in PMNC decreased from 10<sup>3.3</sup> to 10<sup>2.1</sup>. The PMNC from HD patients could not kill the bacteria regardless of the characteristics of the membranes. It was thus concluded that the PMNC of chronic HD patients possess an intact phagocytotic activity which impaired bacterial killing, and was probably due to an abnormality occurring in the ROS production pathway.