Although CD20- relapses of B-cell lymphoma following rituximab therapy have increasingly been reported recently, coexistence of both the original and selected clones on relapse in a single patient have not been described. We experienced such a case with rare CD5+ intravascular lymphomatosis (IVL). A 46-year-old woman was admitted because of IVL complicated with cauda equina syndrome and pulmonary infarction. Complete remission was successfully achieved with multidrug chemotherapy in combination with rituximab. However, the disease recurred after 8 months with leukemic progression and meningeal involvement. The phenotype of the abnormal lymphocytes in the peripheral blood was fundamentally the same (CD20+CD5+CD10-CD19+CD23-sIglambda+) as that of the cells in the cerebrospinal fluid (CSF). However, CD20 expression was decreased remarkably compared with that in the CSF and that in the bone marrow before therapy. The targeting of CD20 molecules on the tumor cell surface by rituximab may have provided a selective pressure on lymphoma cells. The escape phenomenon of the lymphoma cells from rituximab was observed by simultaneously comparing the CD20 expression of cells in the peripheral blood and in a site of sanctuary from rituximab, the CSF.