We reviewed pathologic, phenotypic, and clinical features of extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT) type primarily involving lung to address unresolved questions regarding behavior and pathologic features of unambiguously diagnosed pulmonary MALT lymphoma. Lung specimens from 50 patients were reviewed. Forty-one had low-grade MALT lymphoma. Nine had low-grade MALT lymphoma and diffuse large B-cell lymphoma. The patients included 32 women and 18 men with a median age of 68 years (range 34-88 years). Half of the patients were asymptomatic at the time lymphoma was diagnosed. Radiographic abnormalities were more commonly unilateral (37 patients) than bilateral (12 patients). Localized masses or nodules occurred in 39 patients. Associated autoimmune disorders (29%) and monoclonal gammopathies (43%) were common. Low-grade lymphomas formed intraparenchymal masses composed of centrocyte-like cells, plasmacytoid lymphocytes, and plasma cells that formed lymphoepithelial lesions and exhibited a lymphangitic growth pattern. Mediastinal lymph nodes were involved histologically in 44% of cases. Lymphoma-specific survival was 71.7% at 10 years, and overall survival was significantly worse than age-and gender-matched control patients. None of the following features predicted those patients who had an adverse outcome: systemic symptoms, presence of autoimmune disorders or paraproteinemia, anatomic distribution and number of pulmonary lesions, lymph node involvement, or presence of anthracycline-treated large B-cell lymphoma.