Although monosomy X is the most common karyotype in patients with Turner syndrome, the presence of Y chromosome material has been observed in about 10% of patients. Y chromosome material in patients with Turner syndrome poses an increased risk of gonadoblastoma and malignant transformation. We report a woman with a diagnosis of Turner syndrome at 12 years of age, without signs of virilization, and karyotype reported as 46,X,del(X)(q13). At 26 years, cytogenetic studies indicated the patient to be mosaic for monosomy X and a cell line that contained a du-plicated Yq chromosome. Bilateral gonadectomy was performed and revealed streak gonads, without evidence of gonadoblastoma. Histological analysis showed ovarian stromal cells with few primordial tubal structures. FISH performed on streak gonadal tissue showed a heterogeneous distribution of SRY, with exclusive localization to the primordial tubal structures. DNA extraction from the gonadal tissue showed a 6.5% prevalence of SRY by microarray analysis, contrasting the 86% prevalence in the peripheral blood sample. This indicates that the overall gonadal sex appears to be determined by the majority gonosome complement in gonadal tissue in cases of sex chromosome mosaicism. This case also raises questions regarding malignancy risk associated with Y prevalence and tubal structures in gonadal tissue.