An infected cyst in autosomal dominant polycystic kidney disease was identified with a combined positron emission tomographic (PET) and computed tomographic (CT) system, an experimental setup mimicking an integrated CT-PET scanner. Image fusion of fluorine 18 fluorodeoxyglucose PET and CT images allowed exact localization of the infected cyst among many cysts identified on previous CT and magnetic resonance images. Confirmation was obtained instantly, followed by CT-guided percutaneous puncture. Integrated imaging systems hold promise for direct PET-guided puncture of areas of increased fluorodeoxyglucose uptake by using the anatomic accuracy of CT.