A systematic search for a hidden Y-chromosome mosaicism, in Turner syndrome (TS) patients is justified by the evaluation of the risk of development of germ cell tumors. In this study, we analyzed cryptic Y-chromosome derivatives by polymerase chain reaction (PCR) coupled with fluorescence in situ hybridization (FISH) using Y-specific sequences in patients with TS, and validated this methodology. Unrelated patients with TS (n=32) of Mexican mestizo ethnic origin were diagnosed using cytogenetic analysis. Clinical assessment, endocrine evaluation, karyotyping, FISH and PCR analysis of the Y-chromosomal loci were performed. We found that 9.4% (3 out of 32) patients with TS had Y-chromosome material. Two patients showed Y-chromosome by conventional cytogenetics. One patient had no Y-chromosome by initial karyotyping (45, X) but was positive by lymphocyte PCR DNA analysis of the Y-sequence-specific sex-determining region Y (SRY) gene. Our results suggest that the detection of the Y-chromosome material using sensitive methods, such as PCR coupled with FISH, should be carried out in all patients with TS and should not be limited to TS patients with cytogenetically identifiable Y-chromosome and/or virilization.