The polymerase chain reaction was used to test 18 adults and eight fetuses with numerical or structural X-chromosome abnormalities for the presence of nine Y-specific loci. Y-chromosomal DNA was detectable in one adult patient with X-chromosome mosaicism. This study and previous reports provide evidence to support further screening of patients with X-chromosome abnormalities for the presence of Y-chromosomal DNA sequences. Long-term follow-up of patients with Y-chromosomal sequences is required to determine the risk of gonadal neoplasms and other abnormal phenotypes.