To determine whether quanitative dermal indices are useful in ascertaining the liability for or severity of mosaic Down's syndrome (DS), dermatoglyphics of 107 subjects with proven 46/47,+21 DS were scored by four quantitative dermal indices. The distribution of mosaics by weighted mean percentage of +21 cells ranged from 1 to 95 and was bimodal. Mean maternal age at birth of mosaics (32.9 +/- 7.5 years) was elevated when compared with control maternal ages in the literature. The distribution of quantitative dermal indices for the total mosaic population fell roughly midway between those in the literature for normal and full DS individuals: 73% of mosaics were classified as definitively DS, 21% were in the intermediate range, and 6% were normal. For mosaics who were minimally affected, 24% were DS, 53% intermediate, and 23% normal. One can conclude: (1) For any suspect mosaic, a dermal score in the DS range is highly suggestive of karyotypic pathology. (2) The high prevalence of intermediate scores in normal subjects severely restricts their diagnostic value in screening for mosaics in the general population. For a selected population, such as parents of +21 children, the screening value of quantitative dermal indices remains an open question. Weighted regression analyses demonstrate a highly significant correlation (P less than 0.001) of dermal index score with the weighted mean proportion of +21 cells transformed to the logit scale. One may exploit this correlation to predict the ratio of +21/normal cells in infants, in whom early karyotype evolution can preclude an estimate of the ultimate syndrome based on initial degree of mosaicism. Furthermore, this correlation provides additional indirect evidence that dermal microsymptoms in DS are a reflection of the presence of +21 cells.