Malignant gliomas are invasive into surrounding brain and are refractory to therapy. Telomerase stabilises telomere length and may immortalise cells to allow unlimited proliferation. Our analysis of telomerase activity in 90 human gliomas showed that 19 of 19 oligodendrogliomas and 38 of 51 glioblastoma multiformes have detectable telomerase activity. The absence of telomerase activity in anaplastic astrocytomas (2/20 positive) and in one-quarter (13/51) of the glioblastomas suggests that these tumours follow different pathways of neoplastic progression. Thus we have found that a distinct subgroup of brain tumour consists of transformed yet pre-immortal cells.