The non-invasive continuous inhalation technique of C15O2 and 15O2 coupled with positron emission tomography (PET) provides brain images that are thought to represent local cerebral blood flow (CBF) and oxygen extraction fraction (OEF). Experimental studies in baboons have confirmed that C15O2 inhalation allows tomographic measurement of CBF. The numerous difficulties involved in PET absolute quantitation are stressed, as well as some limitations inherent to the 15O inhalation model. However, the values for local CBF, OEF and CMRO2 obtained in normal young subjects are satisfactory in view of the above-mentioned limitations. The clinical application to recent cerebral infarction has allowed two opposite types of flow-metabolism uncoupling to be identified, which appear to be often predictive if tissue prognosis. The time course of spontaneous changes in CBF and OEF within the infarct is also described. Our studies have, in addition, revealed the previously unknown phenomenon of "crossed cerebellar diaschisis" in supratentorial infarction. Lastly, a state of chronic watershed ischemia, potentially reversible by surgical revascularization, has been identified as presumably involved in the progression of watershed necrosis. The clinical potentials of this method appear considerable.