In order to study the effect of structural chromosomal aberrations on spermatogenesis, quantitative analysis of the germinal epithelium was performed on testicular specimens from 12 carriers of balanced chromosomal translocations diagnosed at male infertility clinics: a Y-autosome translocation, eight reciprocal translocations, and three D/D Robertsonian translocations. Spermatogenic arrest at the spermatocyte stage was revealed in the Y-autosome translocation and four of the reciprocal translocations. The number of spermatids was selectively reduced in three carriers, while normal spermatogenesis was found in three. Thus, the majority of carriers with impaired semen quality showed a complete or partial reduction in the number of spermatids, suggesting that the effect of chromosomal aberrations is to arrest the spermatogenic process at the spermatocyte stage.