The karyotypes of 18 primary 'untreated' gliomas, 6 recurrent gliomas treated with radiotherapy and/or chemotherapy and 2 gliomas before and after treatment are described, based on observations using standard cytogenetic techniques. In comparison to the untreated gliomas there were relatively consistent chromosome differences in the treated gliomas, including (1) deletions of the long arm of chromosome 7 with breakpoint at q22, possibly induced by alkylating agents, and (2) numerous single cell abnormalities or unrelated clones of structural abnormalities, presumably induced by radiotherapy.