We report about a 12-year-old boy with a history of recurrent patella luxation. Due to persistent pain in the distal left femur a MRI examination was performed in another hospital which suggested a malignant bone tumor. Without validation of the MRI findings by conventional radiographs bone biopsy was performed. Histopathological examination yielded the diagnosis of a chondroblastic osteosarcoma. Before initiating polychemotherapy, plain radiographs for the first time, a nuclear imaging study and an additional MRI examination were performed in our hospital. The results of these studies made the diagnosis of an osteosarcoma unlikely. In particular, plain radiographs did not show any osseous lesion which was characteristic of an osteosarcoma. To establish a definite diagnosis biopsy was repeated with resection of the bone area which showed suspicious changes in MRI studies. An osteosarcoma was ruled out by histopathological examination. The pathologic changes detected in MRI were rated as bone bruise on plain radiographs and seemed to be of traumatic origin. Our case report emphasises the importance of conventional radiographs in establishing the diagnosis of an osteoarcoma respectively bone tumors and tumor-like lesions in general. They still remain the mainstay in diagnosing bone forming tumors. MRI imaging studies may show changes which mimick solid lesions but in deed can be of traumatic origin. Without informing the pathologist about the exact origin of the specimen, histopathological examination may lead to the misdiagnosis of a chondroblastic osteosarcoma if specimen, like in this case report, represents epiphyseal tissue showing cartilaginous areas with reactive bone formation.