The relationship between infertility and chromosomal abnormalities has been well documented over the past 25 years [2]. Since an increase in chromosomal abnormalities correlates with a decrease in sperm count, abnormalities in sperm count are the most important indications for chromosome analysis in infertile males [2]. Numerical sex chromosome aberrations constitute a small percentage of the anomalies; since only 3.32% were reported in a study of 2,196 infertile men; and they tend to have a broad spectrum of phenotypic effects [2]. Our patient revealed a 45,X/46,XY mosaicism during routine blood analysis. This karyotype was found in seven cases in the same study [2]. Our patient was infertile because of azoospermia, but he also had a Yq microdeletion covering the AZFb and AZFc regions. It has been reported that numerical Y chromosomal defects may accompany Yq microdeletions, as deleted regions tend to be lost during cell division as a result of mitotic instability [6,7]. There is a close association between large Yq deletions and gonosomal mosaicism in both somatic and germinal cells [5,7]. As in our case, mosaicism in germinal cells may remain undetected unless a specific analysis is performed. We first detected the 47,XYY cell line cytogenetically in testicular tissue and confirmed its presence by FISH analysis in 23% of these cells. The FISH analysis of peripheral lymphocytes revealed the same finding in 8% of cells, which were not identified by conventional cytogenetics. The cases of sex chromosome mosaicism reported in the literature show great diversity, from Turner Syndrome to patients with ambiguous genitalia [9]. The karyotypes with 45,X and 47,XYY cell lines could both present as males and females phenotypically and features like short stature or gonadal tumors have also been reported [9]. Our patient is an apparently normal male, except for the azoo spermia which could be due to the coexistence of a Yq microdeletion. The dominant cell line present in different tissues correlate with the cases reported before. Our patient had different percentages of the 47,XYY cell line in different tissues, as expected in these mosaic states, and this variation may explain the diversity of the phenotypes seen in these patients [3,9]. The formation of 45,X/47,XYY/46,XY mosaicism may be caused by at least two mechanisms. The first is paternal non disjunction at meiosis II followed by loss of the chromosome in subsequent mitoses. The second is a post-zygotic mitotic error which explains the different amounts of mosaicism present in different tissues [3]. Either mechanism could be the cause of mosaicism in our patient, while the presence of Yq microdeletion makes him especially interesting. When sex chromosomal aneuploidy is detected in infertile males, it should be remembered that Yq micro deletions may accompany the clinical picture as they may induce mitotic instability of the Y chromosome, and cause mosaicism that may be undetected in blood cells, but make considerable contribution to the germinal tissue. Evaluation of infertile males with spermatogenetic defects for gonosomal mosaicism could lead to a better assessment for estimating the outcome of assisted reproduction techniques. This case demonstrates the use of testicular biopsy material as another tissue to be karyotyped. The second tissue in this case revealed a third line of cells with gono somal aberration and contributed to predicting an outcome and shaping the genetic counseling for this patient.