Forty-four cases of mesonephric hyperplasia (MH) and two adenocarcinomas arising from mesonephric remnants (MA) in the cervix were compared immunohistochemically with 10 embryonic and fetal mesonephric tissues. The mesonephric cells retained their pattern for intermediate filaments during ontogenesis, as well as in the mature, hyperplastic, and neoplastic states: they expressed cytokeratin 8, cytokeratin 13, and vimentin, the two latter in variable amounts. In embryonic mesonephric tissues, cytokeratin was absent, whereas the staining for vimentin was intense. Fetal mesonephric cells stained for cytokeratin 13 and vimentin, but that staining diminished as maturation progressed. All MH and MA expressed cytokeratin 8, whereas only 20-30% of the cells in MH and 10-20% of carcinomatous mesonephric cells showed positive reactions with anti-cytokeratin 13 and anti-vimentin. CEA was always negative in cells of mesonephric origin. We regard these results to be important, since the reactions with anti-CEA and anti-vimentin enable one to distinguish cervical adenocarcinomas of mesonephric origin from those of endocervical origin, the latter being CEA-positive and vimentin-negative. Clinical studies revealed that approximately 75% of the patients with MH had used oral contraceptives for several years, 46% had precancerous lesions of the cervix, and in 62% the cervical mucosa showed adenomatous microglandular hyperplasia. We believe that hyperplasia of mesonephric remnants in the cervix may occur more often in patients with disturbed hormonal balance. However, the lack of a control population does not enable us to advance this hypothesis with assurance.