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Background: The prognostic importance of extramedullary involvement in patients with Waldenström macroglobulinemia (WM) at diagnosis and treatment options for these patients has not been well evaluated. In this study, we investigated the clinical manifestations, biological features, and effect of first-line therapy on the outcome of WM patients diagnosed with extramedullary involvement (EMWM) vs those with only bone marrow involvement (BMWM). Methods: We analyzed the clinical data of 312 WM patients diagnosed with EMWM (n = 106) and BMWM (n = 206) at The University of Texas MD Anderson Cancer Center from 1994 to 2014. EMWM was confirmed by biopsy, positron emission tomography-computed tomography, or magnetic resonance imaging, and clinical laboratory analyses. Results: Characteristics associated with EMWM were male sex (P = 0.027), age younger than 65 years (P = 0.048), presence of B symptoms (P < 0.001), high serum beta-2 macroglobulin (P < 0.001) level, low serum albumin level (P = 0.036), and cytogenetic abnormalities (P = 0.010). Kaplan-Meier survival analysis results showed that EMWM patients had a significantly shorter median overall survival (P < 0.001) and progression-free survival (PFS) (P < 0.001) than did BMWM patients. Chemotherapy combined with targeted therapy improved PFS for BMWM patients (P = 0.004) but not for EMWM patients. Additionally, initial treatment with rituximab significantly improved the PFS of BMWM patients (P = 0.012) but had no effect on EMWM patients. However, EMWM patients treated with nucleoside analogs attained a better PFS than those who did not (P = 0.021). Conclusions: We show that extramedullary involvement at diagnosis is an adverse prognostic factor in WM patients and that first-line therapy with nucleoside analogs improved PFS for patients with EMWM. The study provides unique clinical and treatment observations in subtypes of WM patient. [ABSTRACT FROM AUTHOR]