Two new cases of phenotypic males with 46,XX karyotype are presented. Fluorescence, autoradiographic and centromeric heterochromatin studies in several lines, including testicular cells, failed to demonstrate the existence of the Y chromosome or the existence of distal Yq material translocated to another chromosome in the two patients. The Xg study in one of the patients and his family provided direct evidence of transmission of an X chromosome from father to son. We present indirect evidence favoring the mutation theory to explain the XX male phenotype.