Conventional intramedullary chondromyxoid fibroma (CMF) is a rare benign tumor of bone; juxtacortical lesions are rarer still, and juxtacortical lesions occurring in children are heretofore essentially unreported. We present a case of such a lesion in a 12-year-old boy. This patient, who was previously healthy, presented with a 1-week history of poorly defined pain and mild swelling in the region of the left proximal tibia. Magnetic resonance imaging and bone scan showed changes most consistent with an aggressive biological process. However, the permanent histologic sections showed a (pseudo) lobular pattern of spindle cells with minimal pleomorphism and other features consistent with CMF. A clonal abnormality was detected in 15% of tumor cells karyotyped, characterized by a break in the long arm of chromosome 6 and a balanced Robertsonian translocation involving chromosomes 14 and 21. The patient has remained well and free of recurrence for more than 4 years. In general, CMF needs to be distinguished from its mimicker low-grade chondrosarcoma, and it must be recognized as occurring on bone surfaces among a wide age range of individuals. Juxtacortical CMF has not proven to be unusually aggressive in adults nor in this child, and marginal (en-block) resection remains the treatment of choice. © 2008 Elsevier Inc. All rights reserved.