The aim of the present study was to investigate whether women with endometriosis displayed a decreased lymphokine-activated killer (LAK) activity. In 15 women with and 7 women without endometriosis the cytotoxicity against four different tumor cell lines – K562, the endometrium carcinomas AN3CA and RL95, the natural-killer (NK)-resistant Daudi cell line – was investigated, using either freshly isolated peripheral blood mononuclear cells (PBMC) or recombinant interleukin (IL)-2-stimulated PBMC. In 5 additional women collagenase-DNase-digested endometrium was used, to investigate whether recombinant IL-2-activated lymphocytes displayed an increased cytotoxicity against fresh and cultured endometrial cells. The cytotoxicity of unstimulated PBMC toward K562, AN3CA and RL95 target cells was decreased in women with endometriosis compared to women without endometriosis (p < 0.05, for all). After recombinant IL-2 stimulation the cytotoxicity toward the four different target cells increased significantly, both in women with and without endometriosis. There was no difference in LAK-mediated cytotoxicity against the four tumoral cells between women with and without endometriosis. Significant LAK activity was demonstrated against both fresh and cultured (72 h) endometrial cells. The cytotoxicity of autologous lymphocytes against cultured endometrial cells was 31.0+ 17 versus 67.4 ± 5.8%, using lymphocytes cultured in medium without and with recombinant IL-2, respectively (paired t test, p < 0.02). These results demonstrated that the decreased NK activity in peripheral blood of women with endometriosis was corrected after in vitro recombinant IL-2 stimulation of lymphocytes. Furthermore, this study showed that cytotoxicity toward fresh and cultured endometrial cells increased after in vitro incubation of PBMC with recombinant IL-2.