Study Design. A case report of transverse stress fracture of the pelvic wing-sacrum after long-level lumbosacral fusion. Objective. To report this rare complication of long-level lumbosacral fusion. Summary of Background Data. There are a number of well-described complications of instrumented lumbosacral fusion, including delayed stress fracture of the pelvis. A bilateral pelvic wing-sacrum transverse stress fracture after long-level lumbosacral fusion has not been previously reported to our knowledge. Methods. Radiography and computed tomography were used to confirm the diagnosis. Long lumbosacral fusion and a pelvic wing-sacrum fracture were shown. Results. A 48-year-old woman underwent several revision spinal surgeries for collapse or instability occurring at the adjacent levels. She presented with low back and bilateral buttock pain with slow progression after last surgery. A bilateral transverse pelvic wing-sacrum stress fracture was found on plain radiographs 7 months later. Conclusions. Stress fracture of bilateral pelvic wing-sacrum can occur as a potential source of late pain after long fusions of the lumbosacral spine. A better understanding of the related biomechanical forces and preoperative risk factors may identify patients at risk and may aid in surgical planning. There are few reports of pelvic stress fracture as a complication of lumbosacral fusion, and it is typically described as a late occurrence. We present the occurrence of a bilateral pelvic wing-sacrum transverse stress fracture, not previously discussed to our knowledge.