Testicular biopsies from eight men with abnormal karyotypes have been examined for histological and cytogenetic evidence of disturbances of meiosis. Quantitative analysis of this material showed one, with a 13;14 Robertsonian translocation, to have apparently normal spermatogenesis. Three patients, one with a 47,XYY and two with 45,XY, inv 9 karyotypes, had an overall depression of spermatogenesis. Four others, all with major chromosomal abnormalities, had apparently normal spermatogenesis until the primary spermatocyte stage. Two of these had sex autosomal translocations. One, 45,Y,t(X;21), had a complete block at MI, the other, 46,X,t(Y;16), had a partial block at spermatid formation. One man with a reciprocal 2;10 translocation showed delay at all stages beyond spermatocyte formation and one man with an inversion of chromosome 3 showed impaired spermatid maturation.