With an incidence of less than 0.3 per cent, post-radiation sarcomas are rare malignant neoplasms with a very poor prognosis. On average, they occur after a latency period of at least 15 years following radiation therapy with doses ranging from 24 to 80 Gy. We present the case of a post-irradiation malignant fibrous histiocytoma (MFH) on the floor of the mouth in a 79-year-old male patient arising only five and a half years after radiation therapy. The primary tumour was classified as a well differentiated squamous cell carcinoma of the right rim of the tongue. Primary therapy was surgical resection of the tumour and post-operative radiation with 50 Gy. Five and a half years later, the patient developed a rapidly progressing MFH within the field of radiation.