Approximately 30% of human astrocytomas have been reported to display allelic loss of the long arm of chromosome 22, suggesting the presence of a chromosome 22q astrocytoma suppressor gene. To define the most likely location for this putative tumor suppressor, we performed deletion mapping on 141 tumors using 16 chromosome 22q microsatellite markers. Allelic loss of 22q was observed in 2/12 (17%) of astrocytomas, 9/29 (31%) of anaplastic astrocytomas, and 38/100 (38%) of glioblastomas, consistent with a role for chromosome 22q loss in astrocytoma progression as well as formation. Twenty-two tumors exhibited allelic loss at every informative locus, consistent with loss of the entire arm of 22q. Twenty-seven tumors showed partial deletions, with one common region of deletion at 22q12.3-q13.1 between markers D22S280 and D22S282, and a second candidate region at 22q13.2 near the marker D22S1170. For the proximal candidate region, the incidence of allelic loss was similar between grades; for the distal locus, the incidence increased with grade, raising the possibility that the distal locus is involved in a later stage of astrocytoma tumorigenesis.