The expression of gap junction connexins (Cx) in the female reproductive tract of rodents and in the human endometrium is highly regulated by steroid hormones. Here we have investigated the distribution and regulation properties of Cx43, Cx26 and Cx32 in the human ectopic endometrium of 41 patients, using immunohistochemistry. The biopsies were obtained during the early or late follicular phase (26 cases), during the corpus luteum phase (five cases) and after a 6 month treatment with a gonadotrophin-releasing hormone (GnRH) agonist (three cases) or progestin (seven cases). Aberrant expression of Cx43 was found in the epithelium of nearly all endometriotic glands whereas Cx26, typical for human uterine epithelium cells, was only detected in 18 cases; in 17 it was co-expressed with Cx43. The stromal compartment of the tissues did not express any connexins investigated. Staining for Cx32 was absent in all endometriotic tissues. Strong expression of Cx43 was correlated with a high serum value of 17 beta-oestradiol, whereas a strong expression of Cx26 was found with high values of progesterone mainly in patients after progestin treatment. The epithelium of endometriotic implants collected after GnRH agonist treatment expressed Cx26 and Cx43 only moderately. The patterns described demonstrate an aberrant connexin expression and a different hormonal regulation pattern in endometriotic tissues compared to the normal cyclic uterine endometrium, thus indicating a high dedifferentiation from the normal situation. However, endometriosis still remains a hormonally-dependent benign disease, and hence, can be treated hormonally.