Lymphocyte surface markers were determined in the peripheral blood lymphocytes (PBL) in 31 stomach cancer patients (15 males and 16 females) and 47 controls (20 males and 27 females) using an indirect immunofluorescence technique. The 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 patients with stomach cancer compared to those in healthy volunteers. In stage I, PBL, none of the PBL subsets nor the CD4:CD8 value were significantly different from those of the controls. In stage II, the numbers of PBL, CD3+, CD4+ and CD8+ cells decreased. In stage III, the CD19+ cells decreased in addition to the decreased subsets in stage II. In stage IV, PBL and all subsets measured decreased. The CD4:CD8 value showed significant increases in stages II, III and IV, because the CD8+ cells decreased to a greater extent than did the CD4+ cells. Changes in the subsets were analyzed with regard to age, sex, performance status and smoking history, no significant relation being observed between these factors and lymphocyte subsets. From the present study, we have demonstrated that lymphocyte subsets were differentially depressed in the order of T cells, B cells and natural killer cells, with progression of the stage of disease.