Electron microscopic examinations were carried out on 29 representative endometriotic glands obtained from 16 biopsies. Patients with endometriosis diagnosed by laparoscopy and confirmed by laparoscopic biopsy prior to hormonal treatment received danazol, 800 mg daily. The tissue samples were obtained after 2, 4, and 6 months of usage of the drug to study the ultrastructural effect of this treatment. The glandular epithelium appeared to be arrested in what looked like the late proliferative stage of the normal menstrual cycle. The ciliated cells remained unchanged for the most part. The findings were correlated with possible effects of danazol on the target tissue, but caution is required in the interpretation of ultrastructural effects of steroids until detailed knowledge of the metabolic pathways is available.