Dermatoglyphic analysis which included topological pattern elements, pattern intensities and dermatoglyphic measurements was performed in 90 male and 110 female fragile X subjects. Data on cytogenetic and intellectual assessments were also available in these individuals. The following conclusions have been drawn from the obtained results. 1. The fragile X syndrome is associated with an appreciable deviation in dermatoglyphic patterns and measurements, especially in males, which may be utilized for diagnostic purposes. 2. The variability in patterns and measurements is considerably increased which, together with irregular distributions, indicates that the fragile X condition may not be homogeneous. 3. The type of deviation in either sex is not inconsistent with the effect of an excessive dosage of the Y, combined with a deficit of the X chromosome and is, therefore, indicative of the X--Y chromosomal rearrangement as an initial lesion in fragile X. Since the deviation may not be specific, direct evidence for such rearrangement should be sought by means of molecular techniques.