Anaplastic thyroid carcinoma (ATC) is a rapidly lethal disease. We present 3 patients with ATC. Patient A was a 68-year-old man, who died before radiotherapy could be started, 4 months after presentation. Patient B was a 74-year-old man who was treated by surgery and radiotherapy. He developed cervical metastasis, which was treated with palliative radiotherapy after which he was lost from follow-up. Patient C was a 55-year-old woman, treated with radiotherapy. She died by euthenasia 7 months after presentation. In adults with a rapidly growing neck lump, malignancy is a frequent cause and should be separated from infectious or reactive causes. Fine needle aspiration with neck ultrasonography are indicated in suspected malignant lumps. ATC is a rare cause of a neck lump. Due to the very fulminant course it should also be considered in an old patient presenting with a rapidly growing lump in the neck.