The most frequent clinical manifestation of anaplastic thyroid cancer is a rapidly enlarging mass at the frontal part of the neck. It is accompanied by severe compression symptoms such as dysphagia and dyspnoea with hoarseness, which could be partially due to paresis of the recurrent laryngeal nerve. We treated a patient with symptoms consistent with a retropharyngeal abscess -swelling of the posterior pharyngeal wall, odynophagia, pain in the left side of the neck, elevated inflammatory markers. The diagnosis of anaplastic thyroid cancer was unexpectedly made after surgery and a subsequent histological examination. Case reportAn 80-year-old woman was referred to Dpt. of Otolaryngology, Head and Neck Surgery, Charles University Medicine Faculty, Teaching Hospital in Hradec Králové from a local hospital where she was treated for suspected urine infection. She suffered from chronic hypertension and insulin-dependent diabetes mellitus and diabetes mellitus was not perfectly compensated. Regarding her family history, there was one relevant piece of information concerning the death of her daughter at the age of 52 from pancreatic cancer.Odynophagia and pain in the left side of the neck appeared during the hospitalization in the local hospital accompanied with significant weight loss. Inflammation markers were significantly elevated (C-reactive protein 239 mg/l, leucocytes 25x10 9 /l). Due to the clinical findings, a computer tomography investigation (CT) was performed and a retropharyngeal abscess was found as well as hypertrophy of the left lobe of the thyroid gland. The existence of a tumour was considered as well and the patient was transferred to Dpt. of Otolaryngology, Head and Neck Surgery, Charles University Medicine Faculty, Teaching Hospital in Hradec Králové.At the time of admission the patient had pronounced swelling of the left side of her neck. It seemed to be the left lobe of the thyroid gland with retrosternal extension. We could also see swelling of the posterior pharyngeal wall, Summary: Introduction: The authors present a rare case of a patient with symptoms consistent with retropharyngeal abscess. The diagnosis of anaplastic thyroid cancer was made after surgery and subsequent histological examination. Case report: An 80-year-old woman was referred to Dpt. of Otolaryngology, Head and Neck Surgery, Charles University Medicine Faculty, Teaching Hospital in Hradec Králové with odynophagia and pain in the left side of the neck. The patient had pronounced swelling of the left side of her neck. We could also see swelling of the posterior pharyngeal wall, more pronounced on the left side. Inflammation markers were markedly elevated. Administration of antibiotics intravenously (amoxicillin combined with clavulan acid and gentamicin) was started. A computer tomography investigation (CT) was performed and a retropharyngeal abscess was found. The existence of a tumour was considered as well. An acute endoscopic examination and a puncture of the retropharyngeal space at the site of the swelling were performed,...