To evaluate the usefulness of autoantibody detection as a nonoperative method of diagnosing endometriosis, we tested 221 sera from 215 patients from the infertility, gynecology, and gynecologic oncology services by an indirect immunofluorescence assay using monolayer cultures of an endometrial carcinoma cell line. The assay showed positive cytoplasmic staining, demonstrating a sensitivity of 83% in the patients with laparoscopically confirmed endometriosis. The specificity of the assay was 65% for the infertility patients and 76% for the infertility and gynecology patients combined. Several gynecologic cancer patients showed either nuclear or nuclear and cytoplasmic staining, but few showed exclusive cytoplasmic staining. These initial results suggest that detection of antibodies might be useful for the diagnosis of endometriosis.