Mediastinitis is a frequently-fatal infection of the connective tissue that surrounds the mediastinal organs. The principal causes are perforation of the oesophagus or infections following thoracic surgery with sternotomy, but it may also occur as a rare but dangerous complication of oropharyngeal or cephalic infections that, spreading through the fascias of the cervical spaces, reach and infect the connective tissue present in the mediastinum and between the pleura. The chief cause of the high rate of mortality still carried by this disease is the poor understanding of this possible complication of oro-facial infections (sometimes initially trivial) and the consequent delay in diagnosis and failure to provide adequate therapy. Mediastinal infections of odontogenic aetiology is a rare occurrence but its management requires an early diagnosis and an aggressive surgical treatment. So all the dentists and the oral surgeons should consider the possibility of onset of this dangerous complication also of banal infections of mandibular molars. The aim of this article is to review the literature, and to report two cases of patients whom, following on to odontogenic infections originating from molars in the mandibular arch, developed an odontogenic cervical abscess complicated by pleural effusion, mediastinal empyema and septic shock, with severe risk of a fatal outcome.