A reappraisal of the coelomic metaplasia theory by reviewing endometriosis occurring in unusual sites and instances.

Endometriosis occurring in unusual sites and instances was reviewed for the purpose of exploring the pathogenesis of the disease. Endometriosis is frequently observed in the pelves of menstruating women. It occurs in teenagers even before menarche, in women who have never menstruated, and in postmenopausal women. It affects not only the pelvic organs but also the right-sided thoracic organs and rarely the extremities. It occurs in men who are castrated and treated with estrogen. The affected sites in both common and uncommon endometriosis include cells arising from the coelomic membrane and sites exposed to metaplasia-inducing substances such as estrogen and unknown factors liberated from the degenerating endometrium. This appears to strengthen the coelomic metaplasia theory, although additional basic and experimental data must still be accumulated to confirm this theory.