Rectovaginal (RV) endometriosis presents with a nodular lesion composed of fibromuscular and endometriotic tissue, and the fibromuscular tissue is the major component in the severe stage. The purpose of our study was to examine the extending process of fibromuscular tissue in RV endometriosis. Histological examinations using immunostains, were performed in 90 RV tissue specimens from 37 women. Fibrosis was present in 89 specimens. In each specimen, the intensity of the fibrosis differed from area to area: in mildly fibrotic areas, the collagen fibers were present around the endometriotic tissue, and in severely fibrotic areas, the fibrosis widely extended into fat and connective tissus as well as within the endometriotic tissue. In the 60 specimens containing endometriotic tissue, the increase in the amount of endometriotic tissue significantly correlated to the increase in degree of fibrosis in the entire tissue. The presence of aggregated smooth muscles, unassociated with blood vessels, was defined as smooth muscle metaplasia (SMM), which was always present within the fibrotic areas, and was observed in 80 specimens. The degree of SMM in the entire tissue was significantly correlated with the degree of fibrosis. From these findings, the following was hypothesized. Initially, endometriotic tissue was present sporadically and fibrosis was present around the endometriotic tissue. Thereafter, proliferation of endometriotic tissue and an increase in fibrosis occur consecutively. The SMM was present within the fibrotic areas, and it became more severe, correlating with the increase in fibrosis. In conclusion, this is the first report describing the extending process of the fibromuscular tissue of RV endometriosis from a histological viewpoint, and we think that recognization of this process is useful for histological diagnosis and clinical management of RV endometriosis.