Finger- and palmar prints of hemi- and heterozygote fragile X-patients with mental retardation (10 males and 5 females) were compared to dermatoglyphic findings in 20 mentally retarded patients (10 males and 10 females) without fragile X and to 200 healthy unrelated persons (100 males and 100 females). Characteristic whorls and double-loops with high ridge-counts on finger-tips and a pronounced transversal course of palmar ridges were restricted to males with fragile X. Female carriers of fragile X showed, corresponding to male patients, some abnormalities of the digital- and palmar ridge-pattern. Contrary to males, in carriers as well as in mentally retarded females without fragile X, fingerprints with low ridge-counts were found. Common to all mentally retarded patients, but more pronounced in males with fragile X, abnormal palmar creases and hand-measurements were observed. These findings probably are related to prenatal retarded growth of the length of the palma and of the middle-finger.