ABSTRACT: General and specific immune function were examined in women with endometriosis. Nonspecific parameters included total leukocyte and differential counts, quantitative immunoglobulin (IgG, IgA, and IgM) determinations, total hemolytic complement, C3, C4, mitogen-induced lymphocyte stimulation, and human leukocyte antigen (HLA) typing; no differences were observed when data were compared to age-matched controls without endometriosis. In contrast, the specific immune response T-lymphocyte-mediated cytotoxicity to autologous endometrial cells was significantly reduced (P <0.01) in women with endometriosis. When results from patients were analyzed according to clinical severity of endometriosis, even more striking immunologic alterations were delineated. In addition, lymphocyte stimulation responses to autologous endometrial antigen were lower in patients with severe or moderate disease, approaching statistical significance (P = 0.18 and 0.12, respectively). These studies suggest an immunologic basis for development of endometriosis.