The present study was conducted in order to investigate the immunologic alterations alongside the numerical changes in peripheral blood lymphocytes(PBL) and their subsets in stomach cancer patients. Lymphocyte surface markers were determined in 85 stomach cancer patients and 49 controls by indirect immunofluorescence technique using monoclonal antibodies. Monoclonal antibodies used were Leu 2a(CD8, suppressor/cytotoxic T cells), Leu 3a(CD4, inducer/helper T cells), Leu 4(CD3, pan T reagent), Leu 11(CD16, natural killer cells) and Leu 12(CD19, B cells). The numbers of PBL, CD3+, CD4+, CD8+, CD16+ and CD19+ cells significantly decreased and the CD4: CD8 value increased in 85 patients with stomach cancer compared to those in controls(p < 0.01). In stage I(n = 17), neither PBL, their subsets nor the CD4: CD8 value were significantly different from those of the controls. In stage II(n = 17), the numbers of PBL, CD3+, CD4+ and CD8+ cells decreased(p < 0.01). In stage III(n = 24) and IV(n = 27), PBL and all subsets measured decreased(p < 0.01). The CD4: CD8 value showed significant increases in stages III and IV(p < 0.01), because the CD8+ cells decreased to a greater extent than did the CD4+ cells. The results demonstrating that the lymphocyte subsets are depressed differentially with the stage suggest that host immunity is impaired with the progression of stomach cancer.