The structural organization of the extracellular matrix of rectal adenocarcinoma of different differentiation degrees without and after neoadjuvant radiation therapy was studied on postoperative material using immunohistochemistry and electron microscopy. The differences in the expression of types I and III collagens, as well as in the ultrastructural organization of the extracellular matrix of rectal adenocarcinoma of different differentiation degrees without and after neoadjuvant radiation therapy were revealed. We observed high expression of collagen I and wide channels in the collagen matrix in the central areas of the well differentiated adenocarcinomas without neoadjuvant radiation therapy and in poorly differentiated adenocarcinomas after neoadjuvant radiation therapy, which can be associated with metastasis and poor prognosis for the patients.