['To investigate the local natural killer (NK) activity of the peritoneal fluid mononuclear cells (PFMC). DESIGN PATIENTS: In a prospective way, the NK activity (K562-assay) was measured in peripheral blood (PB) and peritoneal fluid (PF) of 44 women who underwent a laparoscopy for infertility and/or pain at the University Hospital of Gasthuisberg, Leuven, Belgium.', 'The NK activity of peripheral blood mononuclear cells and PFMC, the number and concentration of PFMC, the percentage of lymphocytes versus macrophages by May-Grünwald-Giemsa staining and the estradiol and progesterone concentration of the PF were correlated together and with the severity of endometriosis.', 'We demonstrated that there is a significant NK activity in PF and that this activity is decreased in women with endometriosis. This defect was more pronounced in the follicular phase of the cycle compared with the postovulatory phase. In PB of the same 44 women, the decreased NK activity correlated with the severity of the disease. This confirms our previous report on another 34 women.'] Is the natural killer activity of peritoneal fluid lymphocytes decreased in women with endometriosis?