We examined peripheral blood T-lymphocyte subsets before initiation of therapy in 79 healthy controls, 3 patients with endometriosis, 95 patients with common epithelial tumors of the ovary, 15 patients with ovarian germ cell tumors, and 3 patients with ovarian sex cord-stromal tumors. In stage Ia/Ib patients with epithelial ovarian cancer, the percentages of activated CD4+ (CD4+HLA-DR+) T cells and activated CD4+ T cells in the CD4+ T-cell subsets were significantly higher than those of healthy controls and patients with benign or borderline epithelial tumors of the ovary. These immunologic parameters were subsequently decreased in patients in stage Ic and more advanced stages. In malignant ovarian germ cell tumors, a similar increase in the CD4+ T-cell subsets was observed. Moreover, the percentage of activated CD8+ T cells in the CD8+ T-cell subsets in stage Ia/Ib patients increased significantly compared with those in healthy controls and patients with benign tumors. Our findings indicate that activated T lymphocytes may play some roles in oncogenesis and progression of both epithelial ovarian cancer and malignant ovarian germ cell tumors.