The lymphocytes and their subsets were determined in the peripheral blood of 62 gastric cancer patients and 68 controls using fluorescent conjugated monoclonal antibodies and flow cytometry (FACScan method). A significant increase in the total number of white blood cells from gastric cancer patients was noted in comparison to controls, but the percentage of lymphocytes was the same. The percentage of suppressor (cytotoxic) T cells (CD8) showed no difference in all stages of gastric cancer patients evaluated, but the percentage of helper (inducer) T cells (CD4) and the CD4/ CD8 ratio decreased significantly in the advanced stages Ill and IV. Depression of the CD4/CD8 ratio was well correlated with tumor invasion, lymph node metastasis, and tumor size but not with sex, age, tumor location, gross type, or histologic differentiation. It appeared that the immune defect of gastric cancer patients was associated with the afferent arm (CD4) and worsened as the disease advanced. These results suggest that immunotherapy to stimulate the deficient immune system may play an important part in the multimodality treatment of patients with advanced gastric cancer.