Infertility affects 10-15% of all couples and the male factor is responsible in approximately half of them. The advances in assisted reproduction techniques and genetic diagnoses have increased the percentage of couples who are able to conceive. We report on an infertile male patient with azoospermia, referred to our laboratory before intra cytoplasmic sperm injection (ICSI), whose peripheral lymphocyte karyotype showed a 45,X/46,XY mosaicism. A Y chromosome (Yq) microdeletion was detected in the azoospermia factor (AZF) regions, AZFb and AZFc. The karyotype from testicular tissue cultures was mosaic 45,X[13]/47,XYY[12]/46,XY[25]. Fluorescence in situ hybridization (FISH) performed on blood lymphocytes and testicular cells showed a 47,XYY cell line in both, in 8 and 23% of cells, respectively. Histological examination of the testicular biopsy revealed very few seminiferous tubules, which were hyalinized and composed only of Sertoli cells. Intracytoplasmic sperm injection was not suggested to the patient due to the sex chromosome mosaicism, Yq micro deletion and biopsy findings. Key words: Male infertility; Sex chromosome mosa icism; Y Microdeletion (Yq)