A Study of 12 Cases YASUAKI NAKASHIMA. MD,' TAKA0 YAMAMURO, MD,t YUZO FUJIWARA, MD,* YOSHlHlKO KOTOURA, MD,t EIGO MORI, M D ,~ AND YOSHIHIRO HAMASHIMA, M D I I Osteofibrous dysplasia (ossifying fibroma of long bones) is one of the fibro-osseous lesions that affects the tibia and fibula in the first decade of life. A study of 12 patients with this lesion showed the high rate of recurrence after surgical intervention. Most commonly, the lesions are eccentrically located in the diaphysis of the tibia. Pseudarthrosis may develop in cases where the lesion is situated in the distal portion. Roentgenologic and histopathologic features are basically similar to those of monostotic fibrous d ysplasia of long bones. However, bony or osteoid trabeculae covered by osteoblasts enable distinction between osteofibrous dysplasia and monostotic fibrous dysplasia, as determined histologically. The current evidence indicates that surgery should not be attempted in patients under 10 years of age.