Objective. Insufficiency fractures of the sacrum are found in women who have undergone radiation therapy to the lower abdomen as well as those suffering from osteoporosis of postmenopausal, steroid-induced, or primary biliary cirrhosis-related origin. Increased up-take in bone scintigraphy and osteolytic changes in these fractures can be misinterpreted as bone metastases, leading to unnecessary biopsies and other procedures in the ensuing search for non-existent primary tumor.Patients. In eight female patients averaging 69.4 years of age, insufficiency fracture of the sacrum was diagnosed by computed tomography (CT) and bone scintigraphy. Three underwent a total of five MRI examinations. Malignancy was excluded by histology in two patients and follow-up of at least 6 months in the remainder. Retrospective analysis of CT scans of 13 patients with metastases in the sacrum revealed no vacuum phenomena.Results. In seven of eight patients with insufficiency fracture of the sacrum, vacuum phenomena were shown on CT examination. The gas was localized centrally within the ventral part of the fracture in three patients; gas was located in ten adjacent sacro-iliac joints of six patients.Conclusions. The vacuum phenomenon may be an incidental finding in osteoarthritis of the sacro-iliac joint, but it has not been previously recognized in IFS. The presence of intra-articular vacuum phenomena in the sacro-iliac joints in combination with a sacral fracture and vacuum phenomena located within the sacral fracture supports a diagnosis of insufficiency fracture or may indeed be the clue by which this diagnosis is established. Insufficiency fractures of the sacrum are a well-known complication in women who have undergone radiation therapy of the pelvis or are suffering from postmenopausal, steroid-induced, or primary biliary cirrhosis-related osteoporosis [1–11]. These fractures are characterized by increased activity on radionuclide bone scans. The lateral masses of the sacrum contain large amounts of hematopoetic bone marrow and are therefore often the site of bone metastasis. Increased uptake in bone scintigraphy in lateral masses of the sacrum and osteolytic destructions on radiographs and computed tomography (CT), especially in patients with a history of a tumor elsewhere, are suggestive of skeletal metastasis, prompting biopsies and additional diagnostic procedures to find a primary tumor [2]. We present seven out of eight patients with insufficiency fractures of the sacrum, in whom we observed vacuum phenomena within the fracture or in the adjacent sacroiliac joints, indicating the benignity of the lesion. This phenomenon has not been recognized previously.