Mutations of the c-KIT gene have been reported not only in gastrointestinal stromal tumors and mast cell tumors, but also in testicular germ cell tumors (TGCTs). In the present study we employed polymerase chain reaction and DNA sequencing analysis to characterize the c-KIT gene in a 29-year-old Japanese patient with bilateral testicular seminomas. Direct sequence analyses revealed a single base substitution in exon 17 in one c-KIT allele, resulting in an amino acid substitution of D816H in this mutated allele. This mutation was found in the left, but not in the right, testicular seminoma. This is the first description of a c-KIT gene mutation in a Japanese patient with bilateral TGCT. The mutational analysis of the c-KIT gene seems to provide crucial information for managing TGCT patients not only in Europe but also in Japan.