Stereotactic radiosurgery is an elegant alternative to microsurgery in both skull base meningiomas and solitary brain metastasis. Though efficacy for both entities has been established prognostic variables pertaining to individual tumour biology have not yet been identified. Therefore the authors measured regional glucose utilisation, thallium-uptake and blood flow in 20 patients (10 meningiomas, 10 metastasis) before and after LINAC radiosurgery. Measurements were performed using SPECT and stable xenon-CT. The mean tumour dose given was 16.3 and 19.5 Gy in meningiomas and metastasis respectively. Metastatic tumours were hypermetabolic in comparison to contralateral normal brain and responders to radiosurgery showed a lower tumour/brain ratio than non-responders (1.43 vs 0.91 respectively, p < 0.01). Meningiomas did not exhibit hypermetabolism and therapeutic outcome was not related to glucose utilisation. Thallium-uptake, however, was closely related to therapeutic response in meningiomas (1.37 vs 2.2 in responders vs non-responders, p < 0.01). This relationship could not be established in metastatic lesions. Blood flow was widely distributed in both meningiomas and metastasis (26-72.8 and 30.2-70.8 ml/100 g/min). rCBF did not correlate with therapeutic outcome. Using FDG-SPECT and thallium-201-SPECT the authors were able to distinguish between tumours likely to respond to stereotactic radiosurgery and those not prone to respond. Furthermore the methodology can be used to monitor therapeutic response in treated tumours before morphologic changes occur.