Four children, a girl and three boys, with diploid/trisomic mosaic Down syndrome were studied for the mechanism of origin of mosaics, using Q- and R-banding heteromorphisms as markers. Three mosaic subjects started as a trisomic zygote followed by the loss of a chromosome 21 at an early mitotic division. Of these, one resulted from a maternal first-meiotic error, another resulted from a paternal first-meiotic event, and the third originated from a first-meiotic error in either parent. The remaining subject could have resulted from either a diploid or a trisomic zygote. These findings, together with a higher proportion of trisomic cells in skin fibroblasts than in peripheral blood lymphocytes in the two patients studied, suggest that the extra chromosome 21 in mosaic Down syndrome patients usually has a meiotic origin. At least two, possibly three, of the diploid cell lines in these mosaics consisted of "uniparental" chromosomes 21, namely, both the homologous members were derived from a parent.