Astroblastomas are rare tumors of cerebral hemispheres of young adults. We report an astroblastoma in a 15-year-old girl and present the first descriptions of cytogenetic abnormalities in this tumor. The tumor was relatively well demarcated from the brain and revealed prominent perivascular rosettes as well as intervascular clear cells that contained abundant glycogen. Cytogenetic analysis revealed an abnormal hypodiploid karyotype with 45 chromosomes and monosomies of chromosomes 10, 21, and 22 and two marker chromosomes in all cells examined. The tumor had a Ki-67 labelling index of 4.7% and assessment of ploidy by flow cytometry revealed 96% of cells in the G0G1 phase and 4% of cells in the G2M phase. Assessment of proliferation and ploidy indices in further cases may provide important prognostic data for this poorly understood entity. Further cytogenetic studies will also help to identify if there are consistent karyotypic abnormalities in these enigmatic tumors.