We identified 382 consecutive patients with lymphoid neoplasms associated with serum monoclonal IgM paraprotein and classified each neoplasm according to World Health Organization criteria. Lymphoplasmacytic lymphoma/Waldenström macroglobulinemia (LPL/WM) was most common, 225 cases (median serum IgM level, 2.2 g/dL; range, 0.2-10.9 g/dL). For 157 cases, classification and median (and range in g/dL) IgM levels were as follows: chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL; n = 77), 0.9 (0.1-2.1); marginal zone lymphoma (n = 27), 0.5 (0.1-2.4); follicular lymphoma (n = 18), 0.4 (0.1-1.6); mantle cell lymphoma (n = 11), 0.4 (0.2-1.3); diffuse large B-cell lymphoma (DLBCL; n = 7), 0.5 (0.2-1.0); DLBCL associated with low-grade lymphoma (n = 5), 0.9 (0.4-3.0); angioimmunoblastic T-cell lymphoma (n = 4), 0.8 (0.8); and CD5+CD23- low-grade B-cell lymphoma, unclassified (n = 8), 0.5 (0.3-2.9). The results show IgM paraproteinemia was associated most commonly with LPL/WM (58.9%), followed by CLL/SLL (20.2%). Although serum IgM levels greater than 3 g/dL were restricted to patients with LPL/WM, most patients with LPL/WM had paraprotein levels less than 3 g/dL. Thus, serum IgM paraprotein level is not a reliable discriminator in differential diagnosis.