Cytogenetic and loss of heterozygosity (LOH) studies of chromosome 5 in male germ cell tumors (GCTs) previously reported suggested the presence of one or more tumor suppressor genes (TSGs) on this chromosome which may play a role in the development of these tumors. In an attempt to further characterize allelic deletions on chromosome 5, we performed a detailed deletion mapping utilizing 66 normal-tumor DNAs from male GCTs assaying 24 polymorphic markers mapped to both the short and long arms. Thirty-seven (56%) tumors exhibited LOH at one or more loci. Loss of one allele at all informative loci was found in 15 of 37 (40.5%) cases suggesting monosomy of chromosome 5. The pattern of LOH in the remaining 22 (59.5%) tumors revealed regional losses identifying three common sites of deletions at 5p15.1-15.2, 5q11, and 5q34-35, respectively. The distribution of allelic deletions was found to be similar in all histologic subtypes with predominance of monosomy in teratomas. Thus, the present study revealed 2 types of chromosome 5 abnormalities in male GCTs, genetic monosomy and regional deletion, the latter identifying three novel sites of candidate TSGs. These data suggest that loss of genetic information on chromosome 5 plays an important role in male GCT development.