The purpose of this study was to evaluate the role of c-fos oncogenes in the development of fibrous dysplasia and osteofibrous dysplasia. The immunohistochemical expression of c-fos protein was evaluated in 15 cases of fibrous dysplasia and 8 cases of osteofibrous dysplasia. Ten cases of fibrous dysplasia were weakly positive with c-fos. Six cases of osteofibrous dysplasia were weakly positive and the remaining two cases were strongly positive. The overall expression of c-fos protein is weaker than high-grade osteosarcoma, thus the implication of c-fos protein is little in the development of these tumors. Fibrous dysplasia and osteofibrous dysplasia share some features of characteristic histology and c-fos expression.