Positron emission tomography (PET) has been applied so far to estimate the viability of malignant tumors or to assess cerebral higher function in living animals and in man. Our preliminary results suggested possibilities of nontraumatic imaging diagnosis of orbital tumors; the rate of incorporation of glucose analogue, 18F-2-fluorodeoxyglucose, seemed to reflect the malignant nature of the tissue. PET also gives information concerning regional blood flow, glucose metabolism and oxygen consumption in the brain. We demonstrated chronic cerebral ischemia in cases with rubeosis iridis. PET was more sensitive in detecting lesions responsible for cortical hemianopsia than X-ray computed tomography (CT) and magnetic resonance CT.