Summary Aim: The aim of our work was to study both the concentration of anticardiolipin antibodies (aCL) in peritoneal fluid in women with endometriosis and to examine peritoneal lymphocyte ability to produce anticardiolipin antibodies. Material and methods: Study group included 30 women with endometriosis. The clinical stages of the disease were assessed by the revised American Fertility Society (rAFS) classification. Reference group included fifteen healthy women, with excluded endometriosis and other pathological disorders within the pelvis. The concentration of aCL in the peritoneal fluid and in fluid from lymphocyte culture was measured by enzyme-linked immunosorbent ELISA assay. Results: Statistical analysis showed significantly increased mean concentration of aCL in peritoneal fluid in women with endometriosis compared to women from the reference group (p<0.0001). The concentration of aCL in fluid from lymphocyte culture was also significantly higher in samples from women with endometriosis than from the reference group (p<0.0001). The highest mean levels of aCL in peritoneal fluid and in fluid from lymphocyte culture were observed in samples from women with stage I of the disease. Conclusions: An increased level of anticardiolipin antibodies in peritoneal fluid in women with endometriosis and increased antibodies production by lymphocytes may suggest an impairment of humoral immunity and its intensification in the early stages of the disease.