The telomeric repeat amplification protocol was used to detect expression of telomerase in primary intracranial tumors. Expression was confined to high-grade variants; 10 of 19 glioblastoma multiforme tumors and 5 of 5 primitive neuroectodermal tumors showed telomerase activity. Two of 8 anaplastic gliomas (1 anaplastic oligodendroglioma and 1 anaplastic oligoastrocytoma) were positive for telomerase. Of 16 meningiomas tested, only the 2 atypical variants were positive for telomerase. Two hemangiopericytomas of the central nervous system also showed expression of telomerase. Twenty-two pituitary adenomas (including 6 invasive variants), 2 low-grade gliomas, 2 ependymomas, and 8 nonneoplastic brain specimens were negative. It was concluded that expression of telomerase is found in most glioblastoma multiforme tumors; primitive neuroectodermal tumors appeared to be more uniformly positive. Expression of telomerase in atypical variants of meningioma and hemangiopericytomas of the central nervous system correlated with the potential for aggressive local growth and metastases. Despite the invasive tendency of some pituitary adenomas, telomerase was not detected in these tumors, which suggests that other pathways account for their aggressive behavior.