Morphological changes in the wall of the seminiferous tubules in the cryptorchid testes were studied using light and electron microscopy. Seventy four unilateral and bilateral cryptorchids (aged 2 to 37 years) were selected, and biopsied specimens were stained with H-E, Azan and Weigert for light microscopic observation. Twenty eight of these specimens were also examined by electron microscopy. In the undescended testes, light microscopy showed irregularity and thickening of the wall (tunica propria), and the thickened wall strongly reacted to the Azan stain. This thickening of the tunica propria was observed to begin at puberty, but was not found in the contralateral scrotal testes of the cryptorchids or in the normal testes. The wall of the normal adult testes was well stained by the Weigert stain, suggesting the presence of elastin. In the undescended testes, however, the wall was not stained in prepubertal or pubertal patients, and even after puberty the wall was stained much less than that in the contralateral scrotal testes and the normal controls. On electron microscopic observation, various ultrastructural changes were noticed in each component of the tubular wall; basement membrane, fibrous layer and cellular layer. In the undescended testes, the lamina densa of the basement membrane consisted of one layer with a serrated outer margin throughout the pre-pubertal and pubertal periods, while after puberty the lamina densa underwent marked multiple lamella formation and increased in thickness. In the fibrous layer was already noticed in the undescended testes of 5-year-old cryptorchids. The development of myoid cells in the cellular layer during puberty was observed to be delayed. The morphological changes occurring in the seminiferous tubular wall of the cryptorchid testes were believed to be related to the impaired spermatogenesis often found in cryptorchidism.