We describe the results of synaptonemal complex (SCs) studies by light (LM) and electron microscopy (EM) in a sterile 46,XY/47,XXY male mosaic. Meiotic studies showed an arrest at the first spermatocyte level. Pachytene figures showed three types of cells: (1) cells with normal SCs, normal sex vesicle, and a 23,XY constitution; (2) cells with no sex vesicle, normal pairing of SCs, and a 24, (?) constitution; and (3) cells with a normal sex vesicle and fragmented SCs. © 1984 Springer-Verlag.