The presence of mosaic 45,X/46,XY is a very rare chromosomal anomaly, with an incidence of about 1.5 per 10.000 in newborn infants and in midtrimester amniocentesis. The phenotype can vary from a normal male to a classical Ullrich-Turner syndrome (UTS). This patients are often infertile. The proposita presented at short stature, primary amenorrhea and hypoplasic uterus. Clinical examination revealed multiple Turner syndrome stigmata. Proposita karyotype was determined as 45,X/46,XY by cytogenetic analysis of peripheral blood. Fluorescence in situ hybridization (FISH) method was performed using centromeric X and Y probe to define the exact mosaicism rate. The rate of mosaicism is found as 45,X(%27)/46,XY(%73) by molecular cytogenetic evaluation. Sex determination was performed using pesudoautosomal region specific primers to establish definitely presence of Y chromosome. In this detection, the case revealed specific bands for Y and X chromosomes. It is impotant to determine definitely mosaicism rate, presence of Y chromosome and sex in UTS patients in terms of prognosis of the disease. It has been presented the importance of the presence Y chromosome in patients with UTS and genetic counseling process labaratory findings in this relatively rare seen case by discussing in view of the literature. ©2004, Fırat Üniversitesi, Tıp Fakültesi