Pulmonary mucosa-associated lymphoid tissue (MALT) lymphoma is associated with pre-existing infections or autoimmune disorders. We report a case of lung cancer initially suspected of MALT lymphoma. The patient was a 73-year-old woman. Complete screening examinations identified a tumor in the right middle lobe. Transbronchial lung biopsy revealed the infiltration of CD20+/CD79a+ lymphocytes invading the structure of the alveolus. MALT lymphoma was suspected, and the middle lobe was resected. The tumor was primarily invasive mucinous carcinoma, and lymphocytic infiltration was observed around the tumor. The monoclonal expansion of B cells and genetic and chromosomal abnormalities which are criteria for the diagnosis of MALT lymphoma were not demonstrated and the lesion was diagnosed as reactive lymphoid infiltrates. Marked lymphocytic infiltration regardless of neoplastic or reactive may suggest the presence of latent lesions.