A 70-year-old man referred for treatment of a left lower lung tumor was shown in chest computed tomography to have a homogeneous round tumor 45 mm in diameter with an enhanced thin wall in the lower lobe of the left lung. No specific finding was seen in material obtained by transbronchial and computed tomography (CT)-guided lung tissue biopsy, so the presumptive diagnosis was a lung abscess. Despite antibiotics administered for 2 weeks, radiography showed the tumor had grown, necessitating left lower lobectomy. The permanent section was diagnosed as diffuse large B-cell lymphoma. Because CT findings for the tumor suggested a lung abscess and the central part of the tumor consisted of fibrotic and necrotic tissue, we had difficulty establishing a final diagnosis. The literature showed primary pulmonary lymphomas yielded a variety of findings radiographically, making surgery paramount for ascertaining a final diagnosis.