A series of 3545 newborn males, born consecutively at a maternity hospital in the western suburbs of Tokyo and with no detectable physical abnormalities, were studied for fluorescent Y-chromatin. Buccal cell smears from each infant were screened. Cases with ambiguous results were subjected to a second test by blood smears, which were found to be more reliable. After the second test, chromosomal analysis was carried out in five infants: three had a 47,XYY karyotype; one, the karyotype 46,XY-D,t(D:Y) (Iijima et al., in preparation); and one, a normal male karyotype. The XYY karyotype occurred in 0.11% of newborn males in this series.