Marked Increase in Serum IgM during Treatment of Waldenström's Macroglobulinemia with Cladribine

To the Editor: We recently encountered two patients with Waldenström's macroglobulinemia 1 who had a marked increase in the serum concentration of IgM M component during treatment with cladribine. 2 Patient 1, a 78-year-old woman, reported shortness of breath and fatigue of two weeks' duration. Physical examination, a complete blood count, and serum chemical values were normal. An IgM-kappa M component was detected in serum; the total serum IgM concentration was 2580 mg per deciliter. A biopsy of a mass in the right side of the chest and right kidney revealed a B-cell lymphoma with plasma-cell differentiation. The bone marrow was also . . .