Monoclonal gammopathy occurs in one-third of the patients with mucosa-associated lymphoid tissue lymphoma (MALT lymphoma). However, monoclonal gammopathy has been rarely reported in Korea. Paraprotenemia accompanying MALT lymphoma is strongly correlated with involvement of the bone marrow, and this involvement leads to the progression of the disease. Here, we present a case of a 66-yr-old man diagnosed with IgM monoclonal gammopathy and stage IV extranodal marginal zone lymphoma of the small intestine, with the involvement of the bone marrow.