The presence of a serum IgD monoclonal protein (M-protein) is usually indicative of a malignant plasma cell disorder. However, one case of well-documented benign monoclonal gammopathy (BMG) of IgD type has been reported. We describe another patient with IgD monoclonal gammopathy of undetermined significance (MGUS) with long-term follow-up. In this patient an IgD lambda M-protein was found on routine serum electrophoresis. She is alive and has no evidence of multiple myeloma or amyloidosis after 8 years of follow-up. Although IgD MGUS is exceedingly rare, this case demonstrates that the presence of a serum IgD M-protein is not necessarily synonymous with a malignant plasma cell process.