A 42-year-old infertile male patient with non obstructive azoospermia was referred to our genetics laboratory for diagnostic studies before ICSI treatment. In prior IVF attempts no mature spermatozoa were found, so the procedure was carried out with spermatid microinjection and failed. The patient had no significant family history and had undergone varicocelectomy 5 years previously. In sperm analysis, the seminal volume was 2.7 mL and no motile or immotile sperm was seen. Chromosome analysis of cultured blood lymphocytes showed a mosaic karyotype 45,X[32]/46,XY[68] and the karyotype interpretation was made according to ISCN 2005 [8]. A Yq microdeletion analysis was performed, screening for two deletions in AZFa region, five in AZFb, four in AZFc and one in AZFd, and for ZFY and SRY genes. The sY277 and sY283 regions of the DAZ gene in AZFc, sY131 and sY143 regions in AZFb, were found to be deleted. Histo logical examination of the testicular biopsy revealed hya linization in the few seminiferous tubules present and they were composed only of Sertoli cells. No germinal epithelium or spermatogenesis was detected. The cells from the testicular biopsy culture showed the karyotype 45,X[13]/ 47,XYY[12]/46,XY[25]. To further investigate the new cell line in blood lymphocytes and to evaluate larger number of cells, fluorescence in situ hybridization (FISH) was performed on both tissues using the centromeric probes, Y chromosome DYZ3 and X chromosome DXZ1 (chromosome X/Y cocktail probe; Qbiogene Inc., Carlsbad, CA, USA). Two hundred cells were analyzed from each tissue and the XYY signal was detected in 8% of the lymphocytes and in 23% of the testicular cells. The patient was informed about his condition during genetic counseling. Due to the sex chromosome mosaicism, Yq microdeletion and biopsy findings, ICSI was not suggested. Sperm extraction from testis was performed, but the procedure produced no spermatids.