CD20 is usually expressed on tumor cells in Waldenström's macroglobulinemia (WM). We report on two patients who achieved good responses with rituximab treatment. <Case 1> A 78-year-old man had anemia and was referred to our hospital in 1997. On admission, IgM-kappa monoclonal protein was detected in the serum and the IgM level was 4850 mg/dl, leading to the diagnosis of WM. In 2002, he developed heart failure due to anemia, and was treated with rituximab. The IgM level decreased to about 200 mg/dl and remained unelevated for 2.5 years. The anemia also improved. < Case 2 > A 59-year-old man was found to have elevated serum IgM (4850 mg/dl) and came to our hospital in 1998. IgM-kappa monoclonal protein was detected in the serum and he was diagnosed as having WM. His IgM level had been controlled with cyclophosphamide administration, but elevated levels were noted again in 2004. He was given rituximab, and a partial response was obtained (IgM 995 mg/dl).