Positron emission tomography (PET) measurement of cerebral blood flow and metabolism has been a basic and standard method for evaluation of hemodynamics in patients with cerebral vascular disease (CVD). Despite the recent rapid spread of PET and PET/CT facilities, the number of patient examinations with (15)O-gas PET scans is declining because most facilities are used for cancer studies. They avoid (15)O-gas PET study because it is time consuming and the technique and calculation methods appear to be complicated. However, reconsidering the benefits and usefulness of conventional (15)O-gas PET study is a good opportunity to understand its potential possibility. Physiological evaluation of cerebral perfusion and metabolism provided the basic concept of hemodynamics in impaired circulation and the development of evidence-based medicine in neurosurgical treatment for CVD. This method can be used for two major objectives, clinical examination with a less-invasive simplified method, and quantitative precise measurements with model analysis for research purposes. Both are important for developing further practical and investigational approaches using (15)O-gas PET.