Osteoporosis is one of the major causes of insufficiency fractures, and since plain radiographic findings are either unhelpful or misleading, insufficiency fracture in the sacrum is particularly difficult to diagnosis and to differentiate from other disease processes such as metastasis. We report the MR findings of two cases of insufficiency fracture of the sacrum. These were not demonstrable on conventional radiographs, but bone scintigraphy revealed unilateral sacral alar uptake, suggesting an active sacral lesion. Both T1 and T2-weighted MR images demonstrated fractures of the sacrum as bands of low signal intensity, with areas of surrounding increased signal intensity seen on the latter suggesting marrow edema.