Stereotactic radiosurgery of meningiomas. Stereotactic radiosurgery has an expanding role in the management of selected intracranial tumors. In an initial 30-month experience using the 201-source cobalt-60 gamma knife at the University of Pittsburgh, 50 patients with meningiomas were treated. The most frequent site of origin was the skull base. Previously, 36 patients (72%) had undergone at least one craniotomy and four patients (8%) had received fractionated external beam radiation therapy. Stereotactic radiosurgery was the primary treatment modality in 16 patients (32%) with symptomatic tumors demonstrated by neuroimaging. Computer imaging-generated isodose plans (with one to five irradiation isocenters) for single-treatment irradiation gave optimal (greater than or equal to 50% isodose line) coverage in 44 patients (88%). The proximity of cranial nerves or vascular, pituitary, and brain-stem structures to the often convoluted tumor mass was crucial to dose selection. Serial imaging studies were evaluated in all 50 patients. Twenty-four patients were examined between 12 and 36 months after treatment; 13 (54%) showed a reduction in tumor volume while nine (38%) showed no change. Of 26 patients evaluated between 6 and 12 months after treatment, four showed a decrease in tumor size while 22 showed no change. Two patients (both with large tumors that received suboptimal irradiation) had delayed tumor growth outside the radiosurgical treatment volume. The actuarial 2-year tumor growth control rate was 96%. Between 3 and 12 months after radiosurgery, three patients developed delayed neurological deficits that gradually improved, compatible with delayed radiation injury. Although extended follow-up monitoring over many years will be necessary to fully evaluate treatment, to date stereotactic radiosurgery has proved to be a relatively safe and effective therapy for selected patients with symptomatic meningiomas, including those who failed surgical resection. Radiosurgery was an effective primary treatment alternative for those patients whose advanced age, medical condition, or high-risk tumor location mitigated against surgical resection. 