Recent data suggest that chromosome mosaicism is a possible mechanism for intrauterine and postnatal survival in cases of trisomy 18 and Turner syndrome (45X). The aim of this study was to evaluate if chromosomal mosaicism is a possible mechanism of survival in Down syndrome (DS) (trisomy 21) individuals. Mosaicism was studied by interphase fluorescence in-situ hybridization (FISH), using a specific probe for chromosome 21 (21q22.13-21q22.2) in 78 cases suspected of DS. To rule out tissue specific mosaicism, buccal cells or amniocytes were analysed in addition to blood in 20 DS cases. Thirty-three per cent of the cases studied by FISH in only peripheral blood were mosaics. In 20 cases of trisomy 21, two tissues were studied and mosaicism was not detected in either of the two tissues in 15 cases. The remaining five cases were mosaics in both the tissues analysed. Clinical comparisons in 17 DS mosaics showed a direct relationship between the percentage of trisomic cells and the degree of phenotypic manifestations. These results suggest that mechanism(s) other than mosaicism may exist for the intrauterine and postnatal survival of DS cases.