A euploid/aberrant double stem line mosaicism was found in two cases of the de Lange syndrome with severe abnormalities. In both cases the structural heterozygosity of the aberrant stem line involved, apparently, the loss of chromosomal material from a smaller autosome of Group (6-12) X, probably No. 11. Differences in the cultural characteristics of de Lange cells suggest that the aberrant stem line may not proliferate in culture, so that mosaicism may not be detected. Moreover, the mosaicism may not be present in all tissues, resulting in normal cytogenetic findings as noted in a third case studied. Our findings suggest that the de Lange syndrome is the phenotypic expression of chromosomal mosaicism.