Serum immunoglobulin (Ig) M monoclonal protein determined by electrophoresis (sIgM-MP) and total serum IgM (sIgM) by nephelometry are widely used for response assessment in Waldenström macroglobulinemia (WM), although have not been compared for predicting changes in underlying disease burden. We, therefore, compared these serum markers with changes in bone marrow (BM) and extramedullary disease for 73 patients who were rituximab naive and treated with a rituximab-containing regimen. By linear regression analysis, reductions in sIgM-MP and sIgM showed moderate correlation with BM disease involvement (r = 0.4051 and r = 0.4490, respectively), and did not differ from one another as estimators of BM disease response (P = .3745). Neither sIgM-MP nor sIgM showed a strong correlation with BM disease response in patients with low (<1000 mg/dL) or high (>5000 mg/dL) IgM levels and extramedullary disease response. sIgM-MP and sIgM, therefore, are comparable response markers in WM. Development of newer, more accurate surrogate response markers are needed to better delineate treatment outcomes in patients with WM and with low or high IgM levels, and extramedullary disease.