A 17-year-old girl was referred to us because of primary amenorrhea and features of Turner's Syndrome. The karyotype obtained from peripheral lymphocytes cultures was mos 45,X/46, XYnf/47, XYnfYnf/46, X, dic (Ynf) (q12). The karyotype of fibroblasts derived from cultures of both gonads was mos 45, X/46, XYnf/47, XYnfYnf. The Y chromosome was non-fluorescent in all the examined lines. We report here the clinical and cytogenetic findings in a patient with an unusual sex chromosome mosaicism.