We report a case of a cervical phlegmone and mediastinitis descending from peridental abscess in 32-year old student. Emergency left thoracotomy was performed after CT evaluation in which multiple fluid levels and gas spaces were found. After 72 hours bacterial strains showed Clostridium perfringens in fluid taken from left pleural cavity and patient was sent to Hiberbaric Center in Gdynia, where he underwent hyperbaric oxygen therapy. After 10 days the patient was brought back in good condition to our Department. After 3 days he died because of sudden massive bleeding caused by necrosis of aorta wall.