We have examined a series of 13 benign and 27 malignant human gliomas for evidence of molecular abnormalities of proto-oncogene and putative tumour suppressor gene loci. The results indicated that specific molecular lesions were associated with increasing grades of malignancy. Thus, loss of genetic material on chromosome 17 was present with approximately equal frequency in both benign and malignant gliomas, whereas loss of loci on chromosome 10 was seen only in malignant gliomas. Only the most malignant tumours, known as glioblastoma multiforme, had more than one molecular abnormality in the same tumour. These findings may contribute to our understanding of glial tumour development, as well as improve the accuracy of tumour diagnosis.