A 76-year-old man was admitted to our hospital with dyspnea. Chest CT revealed bronchial stenosis caused by mediastinal lymphadenopathy. An FDG-PET scan showed increased FDG uptake in the mediastinal lymph nodes, lumbar vertebrae and left ilium. Neither the first biopsy specimen from the left ilium nor the endobronchial lesion showed malignant cells, but both were positive on acid-fast bacillus staining. Genetic testing found the Mycobacterium avium (MAC) gene. Therefore, we diagnosed disseminated MAC infection, and started antibiotic therapy. However, he did not respond to the therapy, and his bronchial stenosis worsened. We performed a biopsy of the newly-appearing supraclavicular lymph node, and of the left ilium again, and confirmed a new diagnosis; diffuse large B cell lymphoma, coexistent with disseminated MAC infection. This case suggests not only the simultaneous occurrence of disseminated MAC infection and diffuse large B cell lymphoma, but also the coexistence of both diseases within the same organs, and that there may be some relationship between the two diseases.