We report the case of a 42-year-old Japanese woman with unusual diaphyseal dysplasia of bilateral femora. Radiographs showed thickening and sclerosis of the cortex with resultant enlargement of the diaphysis, unclear demarcation of the surface of the cortex, and no periosteal reaction. These changes were found on the left femur at the first presentation, and those on the right femur developed within several years. Although this patient partly presented characteristics of Ribbing disease and Camurati-Engelmann disease, the focal involvement of bilateral femora suggested an unknown pathogenesis.