mu-Heavy chain disease (HCD) is a rare monoclonal lymphoid disorder characterized by the failure to assemble a complete IgM immunoglobulin. The mu-heavy chains analyzed to date revealed absence of the variable region and a shortened constant domain. We report the first case of mu-HCD presenting as a benign monoclonal gammopathy. The literature on the 27 reported mu-HCD cases is reviewed, and important clinical and laboratory findings are discussed. The ages of the patients ranged from 15 to 80 years (median, 57.5 years). Twenty-two of 27 patients had an associated lymphoplasma cell proliferative disorder. A monoclonal spike on routine serum protein electrophoresis was found in only 8 of 19 patients. Fourteen of 22 patients had Bence Jones proteinuria, but mu-HCD protein was reported in the urine of only two patients. The survival ranged from less than 1 month to 11 years (median, 24 months).