This paper attempts to place into perspective the present state of clinical investigation with PET in neurology. To this end we briefly review the work of the neurology group of the MRC Cyclotron Unit at Hammersmith Hospital, by reference to studies performed in normal subjects and in patients with cerebrovascular disease. Based on the current understanding of human physiology in health and disease as a result of this new method of clinical measurement in humans, we speculate on some possible future developments and present constraints in the application of PET to clinical research.