The position of 21 trisomy mosaics with an average proportion of trisomic cells approximating 0.5 in relation to normal subjects and those with complete 21 trisomy has been evaluated by means of dermatoglyphic distances, using samples of 142 mosaics, 302 normal controls and 225 complete 21 trisomics for males and females separately and combined. Distances were calculated by means of the simplified D2k method. Penrose's "size and shape" analysis of variance has been applied for comparison to obtain distance coefficients, C2H, based on pattern intensities. Results indicate that mosaics are not intermediate but much closer to 21 trisomics in spite of the fact that the average proportion of trisomic cells in blood approximates 0.5; secondly, that the degree of deviation from the intermediate position of the mosaic sample is roughly proportional to the degree of cytological mosaicism. The position of mosaics is appreciably more intermediate in respect of finger-tip patterns than in respect of palmar and sole loops and triradii. The results obtained here have thus given evidence for the usefulness of dermatoglyphic distances, which enable all the differences in frequencies or means of the respective characters to be conveniently represented by a single number, in studies of the abnormal development of phenotypic characteristics in cases of incomplete trisomy.