We report a patient with mucosa-associated lymphoid tissue (MALT) disease with gastric, bone marrow, and pulmonary involvement and discuss features of this condition that justify its classification and treatment as a lymphoma. The chest radiographic appearance of MALT disease is nondiagnostic and often suggests a range of pulmonary diagnoses. Additionally, while tissue histology may identify features of MALT hypertrophy, there is often difficulty establishing the disease as either benign hyperplasia or low-grade lymphoma on the basis of pathologic criteria. Even when indicators of malignancy are present, the optimal treatment approach may remain unclear. For this reason, decisions regarding treatment are often based on the biologic behavior of a particular patient's MALT disease rather than the strict histologic classification as benign or malignant.