To compare the interleukin-6 (IL-6) gene expression in the peripheral blood mononuclear cells (PBMCs) and plasma IL-6 levels in patients undergoing continuous ambulatory peritoneal dialysis (CAPD) with those in patients undergoing hemodialysis. Eleven hemodialysis patients, 10 CAPD patients, 15 non-dialyzed patients with end-stage kidney disease (ESKD), and 7 healthy controls were included in this study. PBMCs were collected by differential centrifugation. Plasma IL-6 concentration was measured by enzyme immunoassay. Plasma IL-6 levels were significantly increased in the hemodialysis and CAPD patients as compared with non-dialyzed ESKD patients and normal subjects (p < 0.01). Following hemodialysis, plasma IL-6 levels exceeded those before hemodialysis. No significant difference was found in plasma IL-6 levels in CAPD patients and in hemodialysis patients when blood was drawn before hemodialysis. Low but steady-state levels of IL-6 mRNA expression were observed in the non-dialyzed ESKD patients. The expression of IL-6 mRNA in PBMCs was significantly increased in the patients undergoing hemodialysis or CAPD as compared with the non-dialyzed ESKD patients. The PBMC IL-6 mRNA was significantly lower in CAPD patients than in hemodialysis patients (p < 0.01). A significant correlation was found between the plasma concentration of IL-6 and the expression of IL-6 mRNA in PBMCs from patients undergoing hemodialysis or CAPD (p < 0.01). The hemodialysis or CAPD procedure contributed to the increase in PBMC IL-6 mRNA expression and plasma IL-6 concentration. CAPD treatment stimulated the production of IL-6 to a lesser extent than hemodialysis treatment.