Loss of heterozygosity on chromosome 10 (LOH#10) is the most frequent genetic alteration in glioblastomas and occurs in more than 80% of cases. We recently reported that PTEN (MMAC1) on 10q23.3 is mutated in approximately 30% of primary (de novo) glioblastomas but rarely in secondary glioblastomas that progressed from low-grade or anaplastic astrocytomas. Because secondary glioblastomas also show LOH#10, tumor suppressor genes other than PTEN are likely to be involved. We analyzed LOH on chromosomes 10 and 19, using polymorphic microsatellite markers in microdissected foci showing histologically an abrupt transition from low-grade or anaplastic astrocytoma to glioblastoma, suggestive of the emergence of a new tumor clone. When compared to the respective low-grade or anaplastic astrocytoma of the same biopsy, deletions were detected in 7 of 8 glioblastoma foci on 10q25-qter distal to D10S597, covering the DMBT1 and FGFR2 loci. Six of 8 foci showed LOH at one or two flanking markers of PTEN but did not contain PTEN mutations. LOH on 10p and 19q was found in only one case each. These data indicate that acquisition of a highly anaplastic glioblastoma phenotype with marked proliferative activity and lack of glial fibrillary acidic protein expression is associated with loss of a putative tumor suppressor gene on 10q25-qter.