MGUS is characterized by the presence of a serum M-protein less than 3 g/dL; fewer than 10% plasma cells in the bone marrow; no, or only small amounts of, M-protein in the urine; absence of lytic lesions, anemia, hypercalcemia, and renal insufficiency; and, most importantly, stability of the M-protein and failure of development of other abnormalities, MGUS is found in approximately 3% of persons older than 70 years and in 1% of those 50 years or older. During long-term follow-up, approximately one fourth of patients develop multiple myeloma (MM), amyloidosis, macroglobulinemia, or a similar malignant lymphoproliferative disorder. Actuarial rate of development of serious disease was 16% at 10 years, 33% at 20 years, and 40% at 25 years in our experience. The interval from recognition of the M-protein to the diagnosis of MM ranged from 2 to 29 years (median, 10 years). The size of the M-protein, hemoglobin value, percentage of bone marrow plasma cells, amount of light-chain excretion, presence of hypercalcemia or renal insufficiency, and presence of lytic bone lesions are often helpful in differentiating MGUS from MM and macroglobulinemia. The plasma cell labeling index and the presence of circulating plasma cells in the peripheral blood are indicators of active disease; however, there are no findings at the diagnosis of MGUS that reliably distinguish patients who will remain stable from those in whom a malignant condition will develop. Thus, a physician must perform serial measurements of the M-protein in the serum and periodic evaluation of the pertinent clinical and laboratory features to determine whether MM, macroglobulinemia, systemic amyloidosis, or related disorders have developed. Solitary plasmacytoma is characterized by the presence of a tumor consisting of monoclonal plasma cells identical to those in MM. In addition, skeletal roentgenograms must show no lytic lesions, a bone marrow aspirate must contain no evidence of MM, and immunoelectrophoresis or immunofixation of the serum and concentrated urine should show no M-protein. Exceptions to the presence of an M-protein occur, but therapy of the solitary lesion often results in disappearance of the M-protein. Tumoricidal irradiation (4000 to 5000 cGy) for approximately 4 weeks is the treatment of choice. Overt MM occurs in approximately 50% of patients with solitary plasmacytoma. Progression occurs in most patients within 3 years. The three patterns of failure are (1) development of MM, (2) local recurrence, and (3) development of new bone lesions in the absence of MM.