A 49-year-old woman was admitted for an evaluation of exertional dyspnea and general weakness which had persisted for two years. The patient had suffered from dry mouth and dry eyes for five years. The physical examinations showed a coarse breath sound with inspiratory crackles on the whole lung field, particularly on the both basal lungs. The laboratory data disclosed high titers of anti-nuclear antibodies, and anti-SSA (Ro), and anti-SSB(La) antibodies. Chest radiographs demonstrated the presence of bilateral, diffuse, reticulonodular densities in both lungs. Thin-section CT scans showed diffusely distributed mosaic pattern of an inhomogeneous attenuation extending over the entire lung zone. The histological findings from an open-lung biopsy specimen revealed an accumulation of lymphoid cells around the bronchioles and an extension of malignant lymphoma cells from the bronchiolar epithelium toward the alveolar space. Immunohistochemically, the neoplastic cells reacted positively to the CD 20 antigen and were focally positive for the UCHL 1 antigen. The histological diagnosis was consistent with a low grade marginal zone B-cell lymphoma originating in the BALT. Here, we present a case of a histologically proven BALT lymphoma of the lung in a patient with primary Sjogren's Syndrome.