Desmoplastic fibroma of bone is a rare benign tumor that is made up of wavy fibroblasts and abundant collagenous tissue. The case of a 18-year-old patient is presented with a two months history of weightbearing pain in the left knee. Neither native x-ray, CT nor MRI could detect the kind of tumor. The histological findings lead finally to the diagnosis of a desmoplastic fibroma. Wide resection prevented recurrence of the tumor for 8 1/2 years until now. Considering the semi-malignant character of the desmoplastic fibroma and the recurrence rate marginal or wide resection for the primary treatment is recommended. The superior imaging quality of MRI facilitates preoperative planning.