Mucosa-associated lymphoid tissue (MALT) lymphoma is a low-grade B-cell lymphoma and is usually slow to disseminate. The t(11;18)(q21;q21) translocation has been identified in a subset of MALT lymphoma cases, and AP12 and MLT1/MALT1 genes have been implicated in this translocation. However, the clinicopathologic features of t(11;18)-bearing MALT lymphoma have not been fully elucidated, and the optimal therapy for patients with disseminating disease is unknown. We report an outstanding case of MALT lymphoma that showed massive pulmonary infiltration and leukemic transformation during a prolonged course of more than 16 years. Reverse transcriptase-polymerase chain reaction and dual-color fluorescence in situ hybridization analyses confirmed the existence of the AP12/MLT1 fusion gene in the lymphoma cells. Peripheral blood lymphoma cells disappeared after glucocorticoid therapy. Although the pulmonary lymphoma was slowly progressive and caused severe hypoxemia despite the continuation of glucocorticoid therapy, treatment with cladribine induced a marked reduction of pulmonary lymphomatous lesions and an improvement of the hypoxemia. These findings show the unique clinical features of this particular indolent B-cell lymphoma with t(11;18) translocation and suggest the potential therapeutic usefulness of glucocorticoid and cladribine.