The presence of leukocyte subpopulations was demonstrated in peritoneal fluid and in endometrial tissue of women with (n = 16) and without (n = 20) endometriosis. Peritoneal fluid samples were also assayed for effects on lymphocyte proliferation in vitro. Peritoneal fluid leukocyte profiles were observed to be different between these groups. The most significant elevations in total number of leukocytes, macrophages and lymphocytes and natural killer (NK) cells were observed in women with endometriosis. In normal eutopic endometrium T lymphocytes (CD3+), macrophages (Ki-M1+) and NK cells (CD16- CD5+) were present. In contrast aggregates of NK cells (CD16+ CD56-) were only identified in ectopic endometrial tissue. Mitogen-induced lymphocyte proliferation was significantly higher in the presence of peritoneal fluid from patients with severe stage of endometriosis as compared with other samples. Our data indicate that disturbances of the cellular immune system may lead to progression of endometriosis in female peritoneal cavity.