We report an extremely rare case of Turner syndrome mosaicism in a 30-year-old woman. At least 100 metaphases were observed and analyzed through GTG banding with over 550 band resolutions observed. G-banded chromosome analysis revealed a mosaic female karyotype involving 3 different cell lines. One cell line (90% of the analyzed metaphases) presented monosomy X, while 6% of the cells showed trisomy of chromosome 21 and 4% of the cells exhibited a normal female karyotype. Fluorescence in situ hybridization with a locus-specific probe for trisomy 21 and CEP X for monosomy X substantiated the results obtained from karyotyping. Our patient had 2 natural pregnancies, both of which produced children with Down syndrome. In our patient, as is the case with other women with infertility, the necessary routine is cytogenetic analysis (together with genetic counseling). The same analysis can be helpful in implementing assisted reproductive techniques.