The presence of Y-chromosome material in patients with dysgenetic gonads increases the risk of gonadal tumors and/or nontumoral androgen-producing lesions. The patients' prognosis can vary, depending on their karyotype. The objective of this study was to investigate the presence of Y-chromosome mosaicism in Turner syndrome patients and its association with the development of gonadal tumors and/or nontumoral androgen-producing lesions. Eighty-seven Turner syndrome patients were studied. Genomic DNA was extracted from peripheral blood and genes SRY and TSPY and DYZ3 repeat of the Y chromosome were amplified by polymerase chain reaction. To the Y-positive patients, prophylactic gonadectomy was offered. The data disclosed hidden Y-chromosome mosaicism in 16 (18.5%) of the patients. SRY sequence was detected in all of the 16 patients, and 4 (4.6%) of them presented DYZ3 repeat region and TSPY gene. Eleven of the patients with Y-positive sequences agreed to undergo the prophylactic surgery. In 2 cases, bilateral gonadoblastoma was found and, in another case, the histopathologic study of the gonads revealed hilus cell hyperplasia. In a further case, there were hilus cell hyperplasia and a stromal luteoma. In conclusion, a systematic search for hidden Y-chromosome mosaicism, especially SRY, in Turner syndrome patients is justified by the possibility of preventing gonadal lesions.