I've read that the symptoms are a lot alike. But on further thought, and recalling a case of my own many years ago, I realized that in the advanced stages of both diseases, the possibility of confusion over the diagnosis is real. Chlamydia of course is an infection, caused by the bacterium Chlamydia trachomatis. In 80 percent of early chlamydial infections the woman has no symptoms. The infection occasionally causes pain or bleeding during intercourse but most early infections are diagnosed by taking a smear test while performing a routine Pap smear on a woman with no complaints.
Men too are often asymptomatic.
It is at this stage, particularly if the infection has produced an abscess, that it might be confused with endometriosis. Endometriosis in contrast produces pain during a woman's menstrual period right from the beginning. Since this is endometrial tissue, the hormonal cycle of a woman leading up to menstruation causes the endometrial implants to bleed as well, but since they are located outside the uterus the blood cannot escape and is very irritating to the surrounding tissues. This causes the severely painful periods that are typical of this disease. If a young woman has pain during her period that is not relieved by the usual drugs like Motrin or Naprosyn, it is highly likely that she has endometriosis.