Cytogenetic studies on an azoospermic male revealed a balanced Y;1 translocation: 46,X,t(Y;1)(q12;p34.3). In situ hybridization with the probe St35-239 (DXY64) and with a probe detecting telomeric sequences revealed that only the Y telomere is involved in the translocation. Fluorescence in situ hybridization with a chromosome 1 library on meiotic preparations revealed consistent contact of the painted chromosome 1 with the sex vesicle at pachytene, the most advanced stage of spermatogenesis observed. No deletions were observed after Southern blot analysis with probes p49f (DYS1), 50f2 (DYS7), and 52d (DYF27), which map in interval 6 of the Y chromosome, which includes the azoospermia factor (AZF) gene. The results indicate that the infertility of the translocation carrier could be due to an alteration of the sex vesicle structure or to a disturbance of X-chromosome inactivation as a result of the proximity to the autosomal portion.