We have studied the involvement of chromosomal bands 11p13 and 11p15.5 in 15 testicular seminomas (SE) and 18 testicular nonseminomatous germ cell tumors (NS). No allelic imbalances were found in 40% of the SE and 44% of the NS. Loss of heterozygosity (LOH) at 11p15.5 was seen in 21% of the SE and 47% of the NS; the corresponding frequencies for 11p13 were 47% and 44%. Both regions were deleted in 13% of the SE and 44% of the NS, indicating that all NS with a complete LOH of 11p13 also lost the 11p15.5 region. In one (out of two) SE and in five (out of eight) NS, this was due to at least two separate deletions. Loss of the whole p-arm was likely in one SE and two NS. No gross genomic changes of the Wilms' tumor 1 (WT1) tumor suppressor gene were found using a cDNA probe (WT33). Nor were aberrations found in the zinc-finger regions and exons 2 and 6 of this gene, using polymerase chain reaction amplification, single stranded DNA polymorphism analysis, and sequencing. We suggest that loss of genetic information from the short arm of chromosome 11, without affecting the WT1 gene in the regions studied, is relatively frequent but not crucial in the pathogenesis of testicular germ cell tumors of adults.