Cytogenetic and restriction fragment length polymorphism (RFLP) studies have shown that loss of one entire copy of chromosome 10 is a common genetic event in glioblastoma multiforme, the most malignant glial brain tumor in humans. In a search for submicroscopic deletions, we carried out an RFLP analysis using markers that had been mapped accurately on chromosome 10 by genetic linkage analysis. We studied 30 patients of whom 15 had loss of heterozygosity (LOH) at one or more loci. In seven cases, LOH was found at every informative locus, whereas in two cases extensive deletions were observed involving both the short and long arms. In six other patients, LOH was confined to a portion of the long arm. The smallest region of overlap among these latter six deletions was flanked by markers D10S12 proximally and D10S6 distally, a 33.4 centimorgan region that maps physically near the telomere (q25.1-qter). This region will serve as an important target for future mapping experiments designed to identify a tumor suppressor gene implicated in this lethal form of human cancer.