The monoclonal gammopathies include multiple myeloma (MM), monoclonal gammopathy of undetermined significance (MGUS), primary systemic amyloidosis (AL), and Waldenström's macroglobulinemia (WM). At Mayo Clinic, almost 60% of patients with a monoclonal gammopathy have MGUS. MGUS is characterized by the presence of a serum monoclonal protein value <3 g/dL, fewer than 10% plasma cells in the bone marrow, no or a small amount of monoclonal protein in the urine, and absence of lytic bone lesions, anemia, hypercalcemia, or renal insufficiency related to the plasma-cell proliferative process. During long-term follow-up of 241 patients with MGUS seen at Mayo Clinic from 1956 to 1970, MM, WM, AL, or a related disorder developed in 64. To confirm the findings, we conducted a population-based study on MGUS in the 11 counties of southeastern Minnesota from 1960 to 1994. The risk of progression to a malignant plasma-cell disorder was 1% per year.