An 89-yr-old man with a history of right lung adenocarcinoma, left lung squamous cell carcinoma, and penile squamous cell carcinoma presented with lower back pain persisting for 1 mo and general pain and weakness in both lower limbs persisting for 10 days.A bone marrow biopsy revealed a monotonous increase in small lymphocytes with pale cytoplasm and CD20 positivity.To mitigate acute kidney injury with hypercalcemia, treatment with hydration, elcatonin, and zoledronic acid was initiated.Subsequently, rituximab and a reduced dose of THP-COP therapy (cyclophosphamide, pirarubicin, vincristine, and prednisolone) were started.The patient's renal function improved rapidly with the above treatment.Reduction in bone pain also improved his ability to perform activities of daily living.The soluble IL-2 receptor level after the second course of rituximab and THP-COP therapy was 380 U/mL, indicating dramatic improvement in the patient's condition.