INTRODUCTION: Anaplastic carcinoma of the thyroid is a rare but aggressive malignancy which can present with a rapidly enlarging neck mass or compressive sequelae of cough, dyspnoea, dysphagia and hoarseness. Treatment of such tumours is commonly palliative however they occasionally represent surgical challenges due to their rapid growth, diagnostic difficulty and locoregional spread. PRESENTATION OF CASE: A 75 year-old retired veterinary surgeon was referred with a 2 month history of a painless, enlarging neck mass. The patient denied any secondary compressive symptoms or general symptoms of malignancy. On examination a large right-sided neck mass measuring 7cm×5cm was appreciated which was fixed, hard and irregular with associated adenopathy. DISCUSSION: We discuss the diagnostic challenges posed by anaplastic carcinoma of the thyroid and the difficulties in selecting the appropriate intervention in this aggressive disease process. CONCLUSION: Anaplastic carcinoma of the thyroid is encountered infrequently in clinical practice and can generate diagnostic and therapeutic challenges.