A 65-year-old woman was referred to our department because of an abnormal shadow on the chest roentgenogram. The chest roentgenogram showed infiltrative shadows in the right middle and lower lung fields. The chest CT scan showed non-segmental infiltration with a hazy margin and a clear air bronchogram in the right S8 area. A transbronchial aspiration cytology specimen showed atypical lymphocytes, and a transbrochial lung biopsy specimen showed infiltration by atypical lymphocytes. Malignant lymphoma was suspected from the results of the biopsy examination, and a right middle and lower lobectomy was done. The pathological findings showed proliferative atypical lymphocytes in the alveolar and interstitial areas. Malignant lymphoma (diffuse medium cell type by LSG classification) was diagnosed and an immunohistiochemical examination showed monoclonal proliferation of T lymphocytes, which were positive for T lymphocyte surface marker (UCHL-1). Therefore, we believe this was a case of primary pulmonary malignant lymphoma of the T-cell type.