PURPOSE: The purpose of this study was to evaluate the usefulness of MR imaging of pathologic fractures of the long bones. MATERIALS AND METHODS: In 18 patients aged between four and 75 (mean, 25.8) years with histologically confirmed pathologic fractures of the long bones, plain radiographs and MR images were retrospectively analyzed. The former were examined with regard to location and type of fracture, and the presence or absence of underlying disease causing fracture; and the latter in terms of underlying disease, extraosseous mass formation, and soft tissue change. RESULTS: The long bones involved were the femur in nine patients, the humerus in six, and the tibia in three. Underlying diseases were metastatic tumor (n=6), benign bone tumor (n=5), primary malignant bone tumor (n=4), osteomyelitis (n=2), and eosinophilic granuloma (n=1). Plain radiographs showed the fracture site as the metaphysis in ten cases, the diaphysis in five, and the metadiaphysis in one. Fractures were either transverse (n=10), oblique (n=3), spiral (n=1), vertical (n=1), or telescopic (n=1). In two cases, the fracture line was not visible. MR images revealed underlying diseases in all cases. A solid mass was present in all cases of malignant bone tumor, and an extraosseous mass in five such cases. Two benign bone tumors took the form of a cystic mass, two were a cystic mass containing an enhanced solid portion, and one was a solid mass. A soft tissue hematoma was seen in three cases. CONCLUSION: Where pathologic fracture of a long bone had occurred, or a pathologic fracture in which the findings of plain radiography were equivocal, MR imaging was useful for evaluating the pattern and extent of an underlying lesion.