Anaplastic thyroid carcinoma (ATC) is a deadly disease with very limited therapeutic options. There is an urgent need for new and efficacious drugs. Unfortunately accrual in clinical trials is problematic because of the rarity of the disease and often poor performance status at diagnosis. Recently some data have emerged suggesting a role for immunotherapy in the treatment of ATC. We describe the case of a 75-year-old patient with poor performance status and compromised airway and oesophagus at diagnosis, showing a rapid and dramatic response to first line single agent pembrolizumab. Disease progression in the brain occurred 16 months after initial diagnosis. At that time there was ongoing extracranial disease control.