In a 69-year-old female patient a moderately pleomorphic spindle-cell thyroid tumour measuring 5 cm in diameter was initially misinterpreted as primary anaplastic thyroid carcinoma. During clinical investigations to elucidate the cause of severe anaemia, 17 months later an ulcerated duodenal leiomyosarcoma was detected and removed by duodenopancreatectomy. Reevaluation of the thyroid nodule led to revision of the initial diagnosis to metastatic leiomyosarcoma. Six months later the patient died from cerebral stroke. Autopsy findings confirmed the diagnosis of primary leiomyosarcoma of the duodenum with initial manifestation as thyroid metastasis.