Osteosarcomatous differentiation of a phyllodes tumour in the breast is extremely rare. A 54 year old woman presented with a painless lump in her left breast and a mastectomy was performed. Microscopically, the tumour was diagnosed as a malignant phyllodes tumour, where no osteosarcomatous component was found. One year later, two lung tumours were revealed upon chest x ray and were surgically treated. The tumours revealed osteosarcomatous features, and were thought to be metastases from the phyllodes tumour. The present case is very unusual in that osteosarcomatous features are present only in the metastatic lesion of the phyllodes tumour.