Background: The extent and severity of the disabilities is variable among individuals with Down syndrome, although generally characterized by a range of physical and intellectual conditions, including language impairment. Whether the language deficit is due to the intellectual disability (ID) or associated to the supernumerary or portion of chromosome 21 is still debated. Methods: Karyotyping was performed on blood lymphocyte and skin fibroblasts. Fluorescence "in situ" hybridization analysis was performed on cultured lymphocytes and buccal smear cells. Results: The trisomy 21 (T21) mosaicism was characterized by 0.7-10% of mosaic cells in the different tissues, in a 14-year-old girl presenting an intellectual development within the normal range and specific language impairment (SLI) as the only prominent feature. Conclusion: This case illustrates the wide range of phenotypical abnormalities possibly associated with T21 mosaicism. We propose that SLI is indeed a phenotypic trait specific to Down syndrome rather than subsequent to the ID most often associated to the syndrome.