Pyrexia of unknown origin (PUO) and suspected focal infection/inflammation are challenging medical problems. Nuclear medicine methods using scintigraphy with (111)In- or (99m)Tc-labelled antibodies or (67)Ga-citrate have been validated for the diagnosis and detection of inflammatory processes. Recently, positron emission tomography (PET) with (18)F-fluorodeoxyglucose (FDG) has been described as a promising imaging method, especially for PUO. We report the use of FDG-PET in an 18-month-old boy that revealed unexpected infectious colitis after cardiac surgery. This case suggests that FDG-PET is a valuable tool for the detection of unknown inflammatory foci in childhood, especially when the time needed for examination and radiation exposure are to be considered.