Almost all traumatic pulmonary pseudocysts (TPP), such as cavitary pulmonary lesions after blunt chest trauma, resolve spontaneously. On the contrary, secondary infection of a TPP should be considered in the presence of purulent sputum or hemosputum and a persistent cavity. We report a case of an infected TPP that was successfully treated by early surgical treatment. A 25-year-old man was transferred to our hospital with a TPP, shown by computed tomography (CT) as having a thick-walled large cavity, after the acute phase of blunt chest trauma. Purulent hemosputum suggested infection of the cavity. Serial CT scans of the chest revealed a persistent cavity. The thick-walled large cavity was diagnosed as a secondary infection of the TPP, that is, a potential lung abscess. We resected the cavity before a systemic inflammatory reaction occurred.