We describe here a case of monoclonal gammopathy of undetermined significance (MGUS) of IgD type. A trace of serum M-protein was noticed on electrophoresis when a Japanese man visited his primary care physician for medical examination. However, he was asymptomatic. Serum immunoelectrophoresis showed IgD lambda M-protein. Bone marrow examination revealed normocellular marrow with 1.5% plasma cells. Renal function, serum calcium level and complete blood cell count were all within normal limits and a systemic bone survey disclosed no lytic lesions. He was followed for 26 months without chemotherapy, and no disease progression to overt myeloma nor development of systemic amyloidosis resulted. This is, to our knowledge, the third case of IgD MGUS and the first one reported from Japan.