An entity is described which is characterized by low back pain and increased radionuclide uptake in the sacrum at bone scintigraphy in postmenopausal women having received radiotherapy towards the pelvis because of gynaecological malignancy. The findings stimulate bone metastases, but are in all likelihood caused by insufficiency fractures of the sacrum promoted by bone weakness induced by postmenopausal osteoporosis and radiotherapy combined. The increased radionuclide activity has a characteristic appearance which in the fully developed fracture acquires the shape of an 'H'. In plain radiography, changes are absent or subtle. The proper complementary examination is CT, in which the fractures can be visualized and malignant changes excluded. Awareness of this benign entity is important to avoid overdiagnosis of bone metastases.