Internal fixation of fractures in the first few hours following injury has important advantages. It can diminish continued blood loss, improve patient mobility, and may help avoid pulmonary and cardiovascular complications of multiple trauma. Fear of increased risk of fat embolism has limited its use. We studied the release of fat during intramedullary reaming and nailing of the femur in dogs. Comparing fat release from an intact and a fractured femur in each dog, significantly more triglyceride was released from the intact bone than from the fractured femur. Thus, reaming a fractured bone produces minimal embolization. The presence of a fracture may decompress the pressure in the medullary canal, minimizing the release of fat into the circulation during acute internal fixation. This could help to explain the low incidence of clinical fat embolism syndrome associated with immediate intramedullary nailing of the femur.