In 21 patients with cerebral tumours regional cerebral blood flow (rCBF) and blood volume (rCBV) were measured. In the tumours, blood flow and volume were variable and unrelated even in tumours of the same type and grade. In the contralateral grey and white matter a (linear) relationship was found between rCBF and rCBV. Following operation, rCBF and rCBV in the contralateral hemisphere may increase due to lowering of intracranial pressure. Preliminary data shows that both factors decrease following whole brain irradiation.