This study reports on Y chromosomal genotypes of three patients with gonadoblastoma and one patient with gonadoblastoma and mixed germ cell tumor. Molecular analysis for 35 Y chromosomal loci was performed for DNA samples taken from peripheral leukocytes and lymphoblastoid cell lines, showing that the four patients shared the region between DYS267 at interval 4A and DYF50S1 at interval 6D, with the exception of the region around DYS202 at interval 5K. In the patient with gonadoblastoma and mixed germ cell tumor, Y chromosomal material was preserved in the gonadoblastoma but was lost from the mixed germ cell tumor. The results, in conjunction with previous reports, suggest that GBY (gonadoblastoma locus on the Y chromosome) may be located to a roughly 5-Mb pericentromeric region between DYS267 at interval 4A and DYS270 at interval 5A. The presence of Y chromosomal material in gonadoblastoma is consistent with GBY being involved in the development of gonadoblastoma, and the absence of Y chromosomal material in mixed germ cell tumor would be explained as a consequence of Y chromosomal loss from rapidly proliferating gonadal cancer cells.