In the paper showed the case of fatal necrotizing phlegmon of the neck and descending mediastinitis in 48 year old male patient with primary peritonsillar abscess. On the base of literature it was showed the etiology, pathomechanism of the evolution of this heavy complication and surgical and pharmacological methods of its treatment. In spite of prompt wide incision and drainage of phlegmon of the neck and mediastinum and intensive treatment in intensive care unit conditions patient died in the image of septic shock and pus changes in the mediastinum, heart and lungs, what was showed on the base of post-mortum examination.