BACKGROUND: Chondromyxoid fibroma (CMF) is a rare neoplasm of the appendicular skeleton of young adults. We report 20 cases of a poorly recognized subtype which arises on the surface of long bones and erodes the cortical surface causing a periosteal reaction. This entity should be included in the differential diagnosis of bone surface lesions as it may be mistaken for a more aggressive neoplasm. DESIGN: A retrospective review at the Mayo Clinic identified 259 CMF cases, 13 of which were parosteal. Additionally, 2 cases were diagnosed at the University of Alabama at Birmingham and 5 cases were from one of our authors' files. We reviewed the clinical radiographic and pathologic findings of all 20 cases. RESULTS: Juxtacortical CMF occurred over a large age range (12 to 82 y) with a median age of 40.2 years. A slight male predilection (5:4) was seen. The most common presentation was bone pain. All 20 cases showed solitary, radiolucent surface lesions with sclerotic margins and extension into the overlying soft tissues. Most of the lesions were in the proximal tibial metaphysis. Histologically, the tumors had characteristic features of CMF. Several cases contained distinctive areas of calcification, which is not a feature of conventional CMF. Eleven of 12 cases were cured with simple excision. CONCLUSION: CMF should be included in the differential diagnosis of bone surface lesions. The clinical and radiologic findings must be known. The morphology of this lesion is similar to conventional CMF with the exception of focal exuberant calcification. Conservative therapy is the treatment of choice. © 2007 Lippincott Williams & Wilkins, Inc.