Histological observations of 20 occluded vas segments obstructed by the standard procedure of medical polyurethane-vas occlusion in 10 volunteers requesting vasectomy and of 20 vas segments with plugs removed from 10 vas-occluded men requesting vas reversal were carried out in order to elucidate the mechanism of vas occlusion by medical polyurethane (MPU). Twenty vas deferens in vasectomy group were ruptured, only a small amount of MPU elastomer remained within the vas lumen, most having leaked through to encircle the ruptured vas. Histomorphology of the removed vas segments from 10 vas-occluded men, who had undergone MPU-vas occlusion 4 years earlier, showed diffuse proliferation of connective tissue, fibrosis, or hyalinization of fibroplastic tissue and local infiltration of lymphocytes and macrophages. The vas lumen both proximal and distal to the plugs was completely blocked. Sperm granuloma, foreign body granuloma, proliferation of nerve fibers and local infiltration of lymphocytes were found in four vas-occluded men with painful nodules. These results suggest that the contraceptive mechanism of MPU-vas occlusion could be the result of secondary obstruction due to tissue proliferation by MPU irritation after the rupture of the vas deferens.