Abstract The WM1 study is a prospective randomized open-label study that includes patients with previously untreated Waldenström's macroglobulinemia (WM), splenic lymphoma with villous lymphocytes (SLVL), and non–immunoglobulin (Ig) M lymphoplasmacytic lymphoma (LPL) who have an indication for treatment. At registration, patients are categorized as having WM, SLVL, or LPL, and these cohorts are to be analyzed separately. The aim of the study is to compare the efficacy of oral chlorambucil at a dose of 8 mg/m2 (6 mg/m2 for those > 75 years of age) for 10 days every 28 days to a maximum of 12 cycles with oral or intravenous (I.V.) fludarabine at a dose of 40 mg/m2 orally or 25 mg/m2 I.V. (30 mg/m2 orally or 20 mg/m2 I.V. for those > 75 years of age) for 5 days every 28 days to a maximum of 6 cycles. Primary endpoints are response to therapy and duration of response; secondary endpoints are improvement in hematologic parameters, toxicity of therapy, quality of life, and survival. To detect a difference in response rate of patients with WM of 15%, assuming that the overall response rates will be 50% to chlorambucil and 65% to fludarabine, with a power of 80%, requires the sample size of each group to be 183, indicating the need for collaboration among a number of national investigator groups. As of February 2005, accrual to the study stands at 143. Registration, randomization, and data collection are entirely Internet-based (www.waldenstroms.org), and the study is organized by an international collaboration, with a planned interim analysis and an external data monitoring committee.