A 52-year-old man was evaluated in our outpatient facility because of a thoracic mass for one month. A needle biopsy of the chest mass was performed and microbiological culture showed growth of Aggregatibacter actinomycetemcomitans. Three months after starting antimicrobial therapy, acute phase reactants normalized, and chest CT showed a progressive reduction in the size of the phlegmon. To our knowledge, we report the first case of A. actinomycetemcomitans diaphragmatic and chest wall infection without pulmonary involvement. This supports the theory of hematogenous spread of the germ from oral mucosa to produce thoracic lesions.