Background. Multifocal osteosarcoma is usually described as the occurrence of the tumour at two or more sites in a patient without pulmonary metastases and may be synchronous or metachronous. Case report. A previously well 21-year old male, who presented with a swollen, painful right knee with no history of trauma, was found to have a high-grade osteosarcoma of the distal tibia and proximal femur. He underwent resection and prosthetic replacement of the distal femur and proximal tibia and remains well 19 months after diagnosis. Discussion. Multifocal osteosarcoma is a rare condition with a poor prognosis. There is debate about whether it represents multiple primary tumours or metastatic disease.