Objection: to study the origin and morphological substrate of pain syndrome in deep infiltrating endometriosis involving the bowel. Subjects and methods. The investigation was conducted using the intraoperative material (resected portions of the large and small bowels, appendix) obtained from 40 women diagnosed as having deep infiltrating endometriosis involving the bowel, which was accompanied by pain syndrome. Paraffin sections were immunohistochemically examined using the standard protocol. Antibodies to Ki-67, PTEN, ER, PR, ("Dako"), CD34 ("Cell Marque", USA), VEGF, EGF, EGFR, COX-2 ("Abcam"), and MMP 1 and 2 ("Abbiotec") were applied. Dako REAL EnVision Detection System kits ("Dako", Denmark) were used as secondary antibodies. Results. The morphological substrate of pelvic pain syndrome in deep infiltrating endometriosis was established to be factors that acted in situ at the location of endometriotic foci and those caused by the infiltrative perivascular, intravascular, and perineural growth of endometrioid heterotopies. Conclusion. Inflammation and fibrosis in the endometriotic foci contribute to the accumulation of algogenes, which gives rise to somatogenic pain syndrome, and chronic nerve fiber injury as a source of nociceptive stimulation leads to neuropathic pain syndrome.