An azoospermic male was found to have, by means of banding techniques, a 45,X karyotype including a monocentric chromosome 21 with an euchromatic short arm that looked similar to Yp. This rearranged chromosome was further characterized by FISH with a whole Y chromosome paint and the alphoid repeats DYZ3 and D13Z1/D21Z1; the former probe gave a positive signal onto such a peculiar arm without spreading into the long arm, whereas the alphoid repeats revealed an apparent compound centromere with Y- and 21-sequences. Therefore, an unbalanced Y;21 whole arm translocation was concluded and the karyotype written as 45,X.ish der(Y;21)(p10;q10)(wcpY+,DYZ3+,D13Z1/D21Z1+). This patient represents the first case of a Y;21 translocation in an apparent 45,X male, constitutes the fifth instance of a 45,X sterile male, and conforms to previously established karyotype-phenotype correlations.División de Genética, Centro de Investigación Biomédica de Occidente, IMSS, Guadalajara, México Doctorado en Genética Humana, CUCS, Universidad de Guadalajara, Guadalajara, México Hospital General de Occidente, Secretarı´a de Salud, Zapopan, México