Rearrangements concerning chromosome 9p are a late event in the progression of human astrocytic tumors to their most malignant form. The expression of PAX5, which maps to chromosome 9p13, was studied in primary human brain tumors of astrocytic origin. Whereas murine Pax5 is not expressed in the forebrain at any stage, PAX5 expression was increased in a range of astrocytomas (WHO grades II-IV) which originated in the forebrain. Expression of PAX5 was limited to those cells which also expressed the oncogenes myc, fos, or jun singularly or in combination. The epidermal growth factor receptor was highly expressed in glioblastoma multiform tumors in areas which were also highly PAX5 positive. We conclude that the missappropriate expression of PAX5 may aid in promoting the progression of astrocytomal malignancy.