An 80-year-old man had symptoms of urinary retention. Biopsy of the prostate revealed B cell lymphoma. To evaluate the extent of the disease, an F-18 FDG study was performed with positron emission tomography/computed tomography (CT). One hour after intravenous administration of 18.5 mCi F-18 FDG, imaging was carried out from the skull base to the upper thigh. An area of moderately increased activity was noted within the prostate bed. A noncontrast CT, for attenuation correction and anatomic localization, revealed maintenance of fat planes around the prostate, indicating disease localized to that organ. Hence, a B cell lymphoma had been localized to the prostate.