46,XX maleness is characterized by the presence of testicular development in subjects who lack a Y chromosome. The majority of affected persons have normal external genitalia, but 10-15% show various degrees of hypospadias. Several hypotheses have been proposed to explain the etiology of this constitution: translocation of the testis-determining factor (TDF) from the Y to the X chromosome, mutation in an autosomal or X chromosomal gene which permits testicular determination in the absence of TDF, and undetected mosaicism with a Y-bearing cell line. We report the phenotypic data and results of molecular analyses performed in six sporadic Mexican males with 46,XX karyotype. Molecular studies revealed Yp sequences in two individuals (ZFY+ SRY+) with different phenotypes, a third one presented with a smaller segment of Yp (ZFY- SRY+) and complete virilization, while the remaining three were Y-negative and showed hypospadias. In all subjects a hidden mosaicism with a Y-bearing cell line was ruled out due to the absence of Y-centromeric sequences. Our data demonstrate that the phenotype does not always correlate with the presence or absence of Y-sequences in the genome, and confirm that 46,XX maleness is a genetically heterogeneous condition.