A 60-year-old man was admitted for ground glass opacity in the lower lung field and mediastinal lymphadenopathy. Blood tests revealed elevated serum IgM levels, and the urine test detected Bence-Jones protein. Surgical biopsy from the mediastinal lymph node and lung showed small lymphocytes and plasma cells between follicles, and AL kappa amyloid deposition. Genetic examination detected MYD88 L265P mutation. Our diagnosis was lymphoplasmacytic lymphoma (LPL), involving the mediastinum and the lung, followed by amyloidosis. Mutation analysis, in addition to conventional histological evaluation, was useful for a precise diagnosis.