Astroblastomas are uncommon brain tumors whose classification and histogenesis have been debated. Precise criteria for diagnosis have been described only recently, but have not found wide acceptance. We report the clinical, radiographic, and histopathologic features of 20 astroblastomas, and the chromosomal alterations in seven cases as detected by comparative genomic hybridization (CGH). The tumors occurred both in children and young adults (average age, 14 years), most often as well circumscribed, peripheral, cerebral hemispheric masses. Radiographically, the lesions were contrast-enhancing and solid, often with a cystic component. All were characterized histologically by astroblastic pseudorosettes, and most displayed prominent perivascular hyalinization, regional hyaline changes, and pushing borders in regard to adjacent brain. Tumor cells were strongly immunoreactive for S-100 protein, GFAP, and vimentin. Staining for EMA was focal. Ten of 20 astroblastomas were classified as "well differentiated" and 10 were classified as "malignant," largely on the basis of hypercellular zones with increased mitotic indices, vascular proliferation, and necrosis with pseudopalisading. All 10 well differentiated lesions and 8 of 10 malignant lesions were completely resected. None of the well differentiated astroblastomas recurred within the limited follow-up period. Three malignant astroblastomas recurred, including two incompletely resected tumors, and one that had been totally resected. One patient died of disease following recurrence. The most frequent chromosomal alterations detected by CGH were gains of chromosome arm 20q (4/7 tumors) and chromosome 19 (3/7). The combination of these gains occurred in three, including two well differentiated and one malignant astroblastoma. Other alterations noted in two tumors each were losses on 9q, 10, and X. These chromosomal alterations are not typical of ependymoma or infiltrating astrocytic neoplasms, and suggest that astroblastomas may have a characteristic cytogenetic profile in addition to their distinctive clinical, radiographic, and histopathologic features.