Insufficiency fractures of the sacrum were diagnosed during the first year after successful transplantation in four (5.6%) of 71 lung and heart-lung transplant recipients. Each patient had development of low back pain after minor or no trauma; all had osteoporosis. In each instance, plain radiographs failed to demonstrate the fracture, and the diagnosis was established by radionuclide bone scanning that demonstrated the characteristic "butterfly" (bilateral sacral fracture) or "half-butterfly" appearance (unilateral sacral fracture). Sacral insufficiency fractures, a significant cause of low back pain in lung transplant recipients, may be underdiagnosed in this population because routine radiographs do not usually reveal the fracture; bone scanning is the preferred diagnostic modality.