Abstract The coexistence of double aneuploidy of Down and Turner syndromes is rare; most cases have been due to double mitotic errors. The objective of the study was to report a case with monosomy of the X chromosome and trisomy of chromosome 21, in mosaic variety, highlighting the phenotypic effect that the presence of different chromosomal abnormalities can produce and compare with those reported in the literature. A 10-year-old Ecuadorian female, born to a multipregnant mother with 46 years at conception, is seen in consultation with a predominant clinical phenotype of Down syndrome, associated with menarche, presence of pubic and axillary villu, where a karyotype is verified 45 X[7]/47XX+ 21 [3]/46, X, der (X)(: p11.1-> q11.1)[1]/46,XX [1]. The present case is a double Turner-Down aneuploidy, with predominantly X monosomy cell line, who shows important mental retardation and some signs of puberal development not usually in Turner syndrome. These features highlight the clinical importance of doing a karyotype in mental retardation cases and searching low mosaics of another aneuploidies in atypical cases. Its complex chromosomal formula and support with molecular cytogenetics allowed diagnostic confirmation and genetic counseling.