This study investigated CD14(+)HLA-DR(-/low) cells in peripheral blood mononuclear cells (PBMCs) from 64 patients with bladder carcinoma (BC) and 14 healthy controls. Cell phenotypes were determined and CD14(+)HLA-DR(-/low) cells, CD14(+)HLA-DR(+) cells and PBMCs depleted of CD14(+)HLA-DR(-/low) cells were isolated. Proliferation of stimulated PBMCs and interferon-γ (IFN-γ) production after addition of CD14(+)HLA-DR(-/low) and CD14(+)HLA-DR(+) cells at different ratios were measured. IFN-γ production was also measured after addition of L-arginine and/or antitransforming growth factor-β (TGF-β) neutralizing monoclonal antibody, and in PBMCs depleted of CD14(+)HLA-DR(-/low) cells. The proportion of CD14(+)HLA-DR(-/low) cells in BC patients was significantly higher than in controls. CD14(+)HLA-DR(-/low) cells significantly decreased T-cell proliferation and IFN-γ production in a dose-dependent manner. This suppressive activity was partially reversed by L-arginine or anti-TGF-β. Enhanced IFN-γ secretion was also seen in PBMCs depleted of CD14(+)HLA-DR(-/low) cells. The level of CD14(+)HLA-DR(-/low) cells was associated with gender, tumour size, number of tumours, cancer pathological grade and clinical stage. CD14(+)HLA-DR(-/low) cells may represent a subpopulation of myeloid-derived suppressor cells in BC patients.