Cancer patients often manifest psychological or behavioral problems and their brain functions may not always be normal. The purpose of this paper is to present an overview of the results of a series of studies regarding our examination of regional brain glucose metabolism of cancer patients. Following our preliminary study of Japanese cancer patients and a replication in German patients, we explored the possible underlying mechanism of hypometabolism using positron emission tomography with 18F-fluorodeoxyglucose (FDG PET). An increase in the score of Zungs Self-rating depression scale (SDS) was associated with decreased activity in the prefrontal cortex, anterior cingulate gyrus, and striatum. SDS scores and metabolic activity were negatively correlated especially in the basolateral prefrontal cortex. It seemed that the decreased activity in the prefrontal cortex was specifically associated with depressive mood. Further replications have demonstrated that FDG PET could be used for early detection and prediction of the future onset of psychiatric disorders among cancer patients. Recent advancement of magnetic resonance imaging (MRI) has enabled conductance of sophisticated volumetric analysis. Recent studies suggest that cancer patients with intrusive recollections (one of the key symptoms for diagnosing posttraumatic stress disorder) have significantly smaller amygdala and hippocampus. Thus, functional imaging techniques may be helpful in evaluating mild depression in cancer patients.