In the testis, 'hyalinization' of the lamina propria of seminiferous tubules is often accompanied by similar changes within the walls of testicular blood vessels. The aim of our study was to investigate the structure of small blood vessels in hyalinized human testes by means of immunohistochemistry, electron microscopy and image analysis methods. Results of immunohistochemical analysis indicated that, despite hyalinization, testicular small blood vessels retained positive immunostaining for desmin and actin. Their basement membranes remained immunopositive for collagen IV and laminin. No proliferative (Ki-67) activity was observed in the blood vessel walls in testes from both control and infertile men. P-170 glycoprotein was found to be expressed only in primary spermatocytes. No difference in expression and localization of this antigen was observed between control and affected testes. Electron microscopy revealed a number of testicular arterioles with a notably narrow lumen due to enlarged endothelial cells in infertile men. Such arterioles also had a thickened subendothelial layer and an abundant tunica adventitia rich in connective tissue fibres and ground substance. Some venules in hyalinized testes displayed increased connective fibres and ground substance in the subendothelial layer, between the smooth muscle cells of the tunica media and the tunica adventitia. However, no changes were found in the capillary network, when compared to controls. Image analysis data showed a statistically significant increase in the surface of tunica intima and adventitia of arterioles and tunica media of venules. It is concluded that hyalinization mostly affects testicular arterioles and venules, but not capillaries. Our immunohistochemical data indicate that the 'nature' and/or extent of hyalinization in testicular small blood vessels differs from that described previously for the lamina propria of seminiferous tubules.