Fluorodeoxyglucose (FDG) positron emission tomography (PET) has been shown to be useful in the diagnosis and staging of various malignancies. The main advantage of PET is its high sensitivity in identifying malignancies at an early stage. The authors present a patient in whom transthoracic echography and MRI revealed a right atrial mass. The diagnosis was difficult between tumor and thrombus, but PET imaging correctly suggested a malignant tumor.