Our laboratory previously demonstrated that normal human endometrium contains hCG/human LH receptors. Since ectopic endometrial implants in endometriosis arise directly at least in part from uterine endometrium, we investigated whether the implants continue the expression of these receptors. The presence of hCG/LH receptor mRNA and/or immunoreactive receptor protein in ectopic endometrial implants on pelvic peritoneum, uterine endometrium, and unaffected or normal peritoneum from patients with (n = 12) and without (n = 14) clinically apparent endometriosis was examined by in situ hybridization and immunocytochemistry analyses. The results showed that the peritoneal biopsies with visible or microscopic endometrial implants contain receptor mRNA and receptor protein. The glands contain more receptor mRNA and receptor protein than stromal cells in implants similar to uterine endometrium from patients with or without endometriosis. However, there is no consistent difference in the expression of receptors in implants compared to uterine endometria from patients with or without endometriosis. Contrary to ectopic endometrial implants, unaffected or normal peritoneum contain neither receptor mRNA nor receptor protein. In summary, we conclude that ectopic endometrial implants contain hCG/LH receptor mRNA and receptor protein, which suggests new possibilities in the medical treatment of endometriosis.