The majority of the radiofrequency energy emitted by a cellular telephone is absorbed by the hand and head of the user. The total energy absorbed is a function of the specific absorption rate, duration of use, and the manner in which the phone is used. In addition to concerns about potential harmful effects of such exposure, such as the issue of risk of brain cancer, change in brain function related to cell phone radiofrequencies also is of concern. Studies have been conducted to investigate the effect of cell phone use on brain electrical activities, neurophysiology, and behavior. The study by Volkow and colleagues in this issue of JAMA1 is the first investigation in humans of glucose metabolism in the brain after cell phone use.