Waldenström's macroglobulinemia (WM, also known as lymphoplasmacytic lymphoma) is cancer affecting B cells, a type of white blood cell. The main attributing antibody is immunoglobulin M (IgM). WM is an "indolent lymphoma," (i.e., one that tends to grow and spread slowly). It is a type of lymphoproliferative disease, which shares clinical characteristics with the indolent non-Hodgkin lymphomas.

The disease, named after the Swedish oncologist Jan G. Waldenström, was first identified in 1944. As with other lymphomas, the disease is characterized by an uncontrolled increase of B-cells, i.e., white blood cells formed in the bone marrow and lymph nodes. The proliferation of B-cells interferes with the production of red blood cells, resulting in anemia. A unique characteristic of the disease is that the B-cells produce excess amounts of immunoglobulin protein (IgM), thickening the blood, and requiring additional treatment. WM is a rare disease, with only about 1,500 cases per year in the U.S. While the disease is incurable, it is treatable. Because of its indolent nature, many patients are able to lead active lives, and, when treatment is required, may experience years of symptom-free remission.