['To reproduce the earliest phases of endometriosis using a new in vitro model in which cells from a cultured endometrial fragment can proliferate, invade, reconstitute new endometrial-like tissue, and generate blood vessels.', 'Experimental in vitro study.', 'A hospital-based academic research institute.', 'Five normal ovulating women undergoing surgery for various benign gynecological indications.', 'Endometrial samples obtained from the fundus of the uterine cavity were placed in a three-dimensional fibrin matrix culture system.', 'Degree of proliferation of stromal cells and invasion of the fibrin matrix, gland, and stroma formation, vessel sprouting, and immunohistochemical characterization of various cellular components.', 'During the first week of culture, an endometrial cell outgrowth was observed from the original fragments in 120 of 144 wells (83.3%). Subsequently, cell outgrowths could be quantified in 132 (91.6%), 129 (89.5%), and 127 (88.1%) of the wells after 15, 60, and 90 days, respectively. An invasion of the matrix by the human endometrial cells led to the formation of tubular structures that coalesced into tissue, architecturally resembling endometrium and in which the glands were immunohistochemically positive for cytokeratin. New capillaries, immunohistochemically positive for CD31 and vimentin, sprouted from the endometrial outgrowths at the beginning of the fifth week of culture.'] Does three-dimensional in vitro culture of endometrial explants mimic the early stages of endometriosis?