The reported incidence of bone complications after radiation therapy is quite low. The most commonly seen bone complication is insufficiency fractures of the pubis and sacrum. Treatment of insufficiency fractures consists of conservative care, and mineral replacement may be useful. The resolution of symptoms takes at least one year with these treatments. Vascular damage has an important role in the etiology of late radiation injury in normal tissues. Progressive ischemic changes further weaken the bone structure, which can cause fractures, and healing is also delayed. Pentoxifylline is a methylxanthine derivative that is shown to increase tissue blood flow. Here, we present a 63-year-old male patient with pelvic insufficiency fractures due to postoperative pelvic irradiation for rectal adenocarcinoma. The patient received pelvic radiotherapy to a total dose of 50.4 Gy with concomitant 5-FU. Six months after the completion of radiotherapy, the patient presented with severe pelvic pain. Pelvic magnetic resonance imaging (MRI) demonstrated abnormal signal intensity with insufficiency fractures at the sacrum and bone marrow edema near the fractures, but not an abnormal intensity that revealed bone metastases. Neither distant nor locoregional recurrence was observed at his work-up. The final diagnosis was insufficiency fractures of the pelvic bones owing to irradiation, and pentoxifylline (400 mg, 3 times daily, peroral, 1,200 mg/day) was used for eight months as treatment. Dramatic clinical improvement was obtained in six months, and objective healing was revealed with MRI. We concluded that pentoxifylline is a cost-effective drug with minimal adverse effects in treating radiation damage of bone.