Finger prints of a series of 48 Japanese patients with Klinefelter's syndrome (47, XXY) were studied as to frequencies of patterns and ridge counts. The patients displayed frequencies of the finger patterns on all digits similar to those of normal female controls. The finger patterns on individual digits of the patients, nevertheless, showed frequencies distinguishable from those of both normal male and female controls; arch pattern on digit I was significantly higher in incidence in Klinefelter's syndrome, whereas on digit II, ulnar loops were significantly more frequent and whorls less frequent in the patients. The patients were also noted to show significantly lower ridge counts for loops and whorls as compared with the normal controls. In Klinefelter's syndrome, significant diminution of the total finger ridge counts was demonstrable and this has been attributed not so much to the increased incidence of arches as to the frequent occurence of patterns with low ridge counts. The frequencies of finger patterns on all digits similar to those of normal females, the increased occurrence of patterns with low ridge counts as compared with the normal females, noted in the present series, are the characteristic features that are in common with those reported on white patients with this syndrome.