A 68-year-old man was admitted to our hospital with left intermittent claudication. Computed tomography showed soft tissue masses surrounding the left iliac artery and in the bilateral pulmonary hilum, and the first FDG PET showed increased FDG uptake by the lesions. Retroperitoneal fibrosis associated with mediastinal fibrosis was most suspected. An open biopsy of the left peri-iliac masses revealed retroperitoneal fibrosis. Corticosteroid treatment was initiated. The second FDG PET under corticosteroid treatment showed no pathological FDG uptake. The third FDG PET after cessation of corticosteroid treatment showed increased FDG uptake in the mediastinum, and so Sairei-to treatment was initiated. The fourth FDG PET under Sairei-to treatment showed no improvement of the pathological FDG uptake, and so low-dose corticosteroid was re-started in combination with Sairei-to treatment. The fifth FDG PET under Sairei-to and corticosteroid treatment showed no pathological FDG uptake. These FDG PET findings suggest the usefulness of FDG PET for the diagnosis and monitoring of retroperitoneal fibrosis associated with mediastinal fibrosis.