ABSTRACT:Cellular telephones are becoming increasingly common; their small size and portability means that they may be unrevealed when a patient has an EEG. They contain radio-frequency receivers and transmitters, and may produce EEG artifacts by several mechanisms. We have ascertained three patterns of EEG artifact that may be generated by cellular telephones: an“autonomous registration”artifact that may occur with the telephone turned on, but not otherwise being used, and“sending”and“receiving”artifacts which may be seen with use of the cellular telephone. The artifacts have a characteristic appearance. Awareness of these artifacts should prompt the technologist to determine if a cellular telephone is present in or near the recording room.