The in vivo generation of proinflammatory cytokines by monocytes is thought to be enhanced in hemodialysis (HD) patients; this process may be associated with acute and chronic complications seen in these patients. However, the acceptability of this hypothesis in view of modern HD treatment is questionable. We addressed this issue by examining cytokine production on a single-monocyte basis using flowcytometry. Eleven maintenance HD patients (mean age, 55±2.3 years old) and 10 age-matched healthy controls were enrolled in the study. The HD patients underwent dialysis treatment using biocompatible HD membranes, such as polysulfone (eight patients) and cellulose triacetate (three patients), and a purified bicarbonate dialysate (containing an endotoxin concentration of less than 1.0EU/L). Two milliliters of peripheral blood were incubated with 15μg/mL of Brefeldin-A and 0.1μg/mL of lipopolysaccharides for 4 hours at 37°C in an incubator under 5% CO₂. Monocytes were labeled with a monoclonal antibody to CD14 (Immunotech), and their intracellular cytokines, such as interleukin (IL)-1β, tumor necrosis factor (TNF)-α, IL-6 and IL-8 were stained using FASTIMMUNE intracellular cytokine staining kits (Becton Dickinson) according to the manufacturer's protocol. The percentage of cells that stained positive for each cytokine was simultaneously analyzed on a single-cell basis using a flow cytometer (Coulter Electronics). The plasma levels of these cytokiness were also measured in the same blood samples used for intracellular analysis. The following results were obtained: (1) There were no significant differences in the intracellular and plasma levels of monokines between the two groups with the exception of plasma IL-8 and IL-6. (2) HD treatment significantly increased the intracellular production of TNF-α and IL-8, but did not affect any plasma monokine level. In conclusion, the present results suggest that the original interleukin hypothesis does not explain the current state of HD patients who are treated using biocompatible membranes and purified dialysates; however, the present findings do suggest that this hypothesis was true prior to these recent advances in hemodialysis. Activated TNF-α and IL-8 synthesis following HD treatment could be involved with complications commonly observed in chronic HD patients.