Chromosome studies on the parents and sibs of subjects identified with numerical sex chromosome abnormalities through a sex chromatin and Y body survey of Toronto newborns have been performed. In 51 full sibs of 30 probands there were one XXY brother and one XYY brother of two different XXY subjects. No parental mosaicisms were demonstrated, although these were looked for in the parents of these two families. While no other aneuploidies were found in these sibs, other reports strongly suggests that the risk to sibs includes risk of autosomal anomalies as well.In addition to these findings in sibs, two of the XXY probands were found to be half-first cousins.