Allelic combinations at seven loci on human chromosome 17 defined by restriction fragment length polymorphisms were determined in tumor and normal tissues from 35 patients with gliomas. Loss of constitutional heterozygosity at one or more of these loci was observed in 8 of the 24 tumors displaying astrocytic differentiation and in the single primitive neuroectodermal tumor examined. (16), and here we have applied such analysis to a series of gliomas to examine clonal genotypic changes in these tumors. Our data indicate that loss of sequences on chromosome 17p, an aberration that has not been cytogenetically associated with any glioma subtype, is the only aberration we have detected in all adult malignancy grades of astrocytoma, is the most frequently observed aberration in tumors of malignancy grade II and III of this glial cell subtype, and appears to be specific to gliomas displaying solely astrocytic differentiation. Furthermore, these results indicate that the loss of chromosome 17p sequences most frequently involves mitotic recombination. As has been previously demonstrated with embryonal rhabdomyosarcoma (17), mitotic recombination mapping offers an approach toward high resolution subregional localization of loci involved in the evolution of specific tumors. The data presented here corroborate the value of this approach by revealing a region of the short arm distal to a marker in band p11.2 that attains common homozygosity for those astrocytomas losing genetic information on chromosome 17 and, thus, is inferred to harbor a locus or loci whose rearrangement is related to the development of these tumors.