An odontogenic myxoma of the maxilla with an aggressive clinical course is presented. The tumour arose in a 53-year-old patient, recurred two times after extended maxillectomy and ultimately caused the patient's death by uncontrollable local disease with infiltration of the cranial cavity. Microscopically, the tumour showed histological features of a low grade malignant myxosarcoma with cellular areas, enhanced mitotic activity and nuclear pleomorphism. Cytogenetic analysis revealed an unexpectedly aberrant hypertetraploid chromosome complement, that was considered as incompatible with the usual karyotypic patterns of benign tumours.