A 32-year-old woman was referred to our hospital due to systemic lymphadenopathy. The patient's peripheral blood showed expansion of CD5+CD20+ CD38+ CD23- mature lymphocytes. However, the axillary lymph nodes were infiltrated by both CD23+ large lymphocytes and CD23- small lymphocytes. Because the pattern of the rearranged immunoglobulin heavy chain gene was different between the peripheral blood and lymph node samples in a Southern blot analysis, the patient was diagnosed with Richter syndrome, in which diffuse large B-cell lymphoma develops from a clone distinct from B-cell chronic lymphocytic leukemia. After undergoing rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone (R-CHOP) therapy, the patient was successfully treated with allogeneic hematopoietic transplantation, and no relapse was observed for three years.