Monoclonal gammopathies of undetermined significance constitute a body of asymptomatic diseases characterized by a selective increase of a single serum immunoglobulin due to an only dysregulated clone of B lymphocytes. They occur in up to 10% of persons over 80 years of age. The diagnosis is suggested in presence of both M component in serum <30 g/L and bone marrow plasma cells <10% by the exclusion of multiple myeloma or lymphoproliferative disorders. In most MGUS, however, populations of normal and myelomatous plasma cells coexist in a stable ratio. The overall relative risk for progression is 7 times the expected rate, depending from the disease and the type of M component.