A novel subpopulation of blood monocytes coexpressing CD16 antigen and low levels of CD14 antigen (CD14+CD16+ monocytes) has recently been identified, and expansion of these CD14+CD16+ monocytes has been reported under some pathological conditions. In this study, we examined the immunophenotype of blood monocytes in patients with chronic renal failure (CRF) who were undergoing hemodialysis (HD, n = 52) or continuous ambulatory peritoneal dialysis (CAPD, n = 36) using two-color immunofluorescence flow cytometry. The percentage and absolute number of CD14+CD16+ monocytes were significantly higher (p < 0.001) in both HD and CAPD patients compared with those in healthy control subjects. We also determined the plasma concentrations of hematopoietic growth factors and cytokines using an enzyme-linked immunosorbent immunoassay. The plasma levels of macrophage colony-stimulating factor (M-CSF) were markedly increased in both HD and CAPD patients relative to the normal controls. The plasma M-CSF levels correlated significantly with the number of CD14+CD16+ monocytes in the whole group of subjects. These findings suggest that elevated endogenous M-CSF levels may participate in the expansion of CD14+CD16+ monocytes in CRF patients undergoing dialysis.