We examined the lymphocytes of peripheral blood (PB) and peritoneal fluid (PF) of women with or without endometriosis to investigate the alteration of cytotoxic activity of natural killer (NK) cells and activation of T cells in the peritoneal cavity of women with endometriosis. A total of 16 control women and 14 patients with stage III-IV endometriosis were selected on the basis of laparoscopic examination in National Taiwan University Hospital. The lymphocyte subpopulations (B cell, NK cell and T cell), including T-cell activation markers (CD69, CD25, HLA-DR), in PB and PF were analysed by dual-colour flow cytometry. The NK cytotoxicity of PB and PF mononuclear cells was evaluated by 51Cr release assay. There was a significant decrease of NK cytotoxicity and CD25+ CD3+ lymphocyte subpopulation in PF of women with endometriosis compared with those without endometriosis. However, there was no difference in the proportion of NK cells in both PB and PF between women with and without endometriosis. Therefore, the decreased NK cytotoxicity in PF of women with endometriosis was due to the functional defect, but not quantitative defect, of NK cells. The concomitant reduction of activated T cells in women with endometriosis might suggest its possible role in the defect of NK cytotoxicity.