Testicular histology in adult men with Klinefelter's syndrome (KS) is characterized by degenerative changes of the seminiferous epithelium. In contrast, limited changes have been reported in XXY fetuses. However, knowledge about the natural history of the degeneration of testicular histology in KS is scarce, and similarly testicular histology in prepubertal individuals with XY/XXY and XYY karyotypes is described incompletely. We have performed a qualitative and quantitative study of testicular histology in 11 XXY, one XY/XXY mosaic and two XYY boys between the neonatal period and 13 years of age using stereological methods and control data from normal XY boys and XY boys undergoing surgery for testicular maldescent. Testicular specimens were obtained at autopsy (n=2) or at time of surgery for malposition of the testis (n=10) or for inguinal hernia (n=2). The seminiferous tubules showed no signs of degeneration in any of the specimens. The volume density of seminiferous tubules was normal at all ages, whereas mean tubular diameter was reduced in all but three XXY boys. Germ cell numbers were only normal in XXY and XYY infants as well as in the 12 year old mosaic. No germ cells were observed in any XXY boy age 2 years or more. Leydig cells were observed in the infants and in a 13 year old XXY boy. The changes were comparable to what has been found in the most severely affected XY boys with testicular maldescent. We conclude that testicular histology in individuals with sex chromosome aneuploidy seems to be near normal during infancy, after which time degenerative changes occur.