We report the fluorodeoxyglucose positron emission tomography (FDG-PET)/computed tomography (CT) findings of autoimmune pancreatitis (AIP) associated with idiopathic retroperitoneal fibrosis. A 69-year-old male patient was admitted to our hospital with obstructive jaundice. Six months prior to this admission, he was treated with steroid therapy for retroperitoneal fibrosis. Laboratory data showed that elevated T-bil, C-reactive protein, amylase and immunoglobulin 4, and antinuclear antibodies were positive. Clinical history, laboratory data, CT image, and magnetic resonance imaging led to a diagnosis of autoimmune pancreatitis. To investigate the inflammatory activity, FDG-PET/CT was undertaken. FDG-PET/CT demonstrated diffuse intense FDG uptake in the enlarged pancreas and diffuse mild uptake in the region of the abdominal aorta-bilateral iliac arteries. A dilated right renal pelvis and upper ureter, corresponding to hydronephrosis probably caused by retroperitoneal fibrosis, were shown. An FDG-PET/CT was useful to evaluate inflammatory activity and morphological imaging, and supported our diagnosis of AIP and retroperitoneal fibrosis.