A 56-year-old male was admitted to our hospital with lymphocytosis (16.4 x 10(9)/l; 79% lymphocytes including 50% small lymphocytes), generalized lymphadenopathy, massive splenomegaly, and heavily infiltrated bone marrow. Immunophenotype analysis of the neoplastic cells in the bone marrow revealed that they were B cells (CD20 + CD19 + Ia1 + sIgM+) positive for CD10. By contrast, the cells in the lymph node were CD20 + CD19 + Ia1 + sIgM+ but negative for CD10. The patient was tentatively diagnosed as having lymphosarcoma cell lymphoma, however, the final diagnosis was leukemic phase of intermediate lymphocytic lymphoma. We concluded that CD10+ neoplastic cells in the bone marrow and peripheral blood had differentiated to CD10- cells.