A 66-year-old woman with severe congestive heart failure, leucocytosis and recent cerebral thrombosis, was admitted to our hospital. Examination of her heart revealed mitral steno-insufficiency, tricuspid regurgitation and left atrial thrombi. The hematological study confirmed the diagnosis of chronic neutrophilic leukemia (CNL). Administration of carbazilquinone (0.5-0.75mg/day) was quite effective to stabilize CNL. However, repeated episodes of cerebral thromboembolism, intractable heart failure and patient's desire pushed us to perform cardiac operation. Utilizing routine cardio-pulmonary bypass and moderate hypothermia, removal of left atrial thrombi, mitral valve replacement with a 27mm Björk-Shiley prosthesis and tricuspid annuloplasty with a 32mm Carpentier-Edwards prosthetic ring were performed. We had no difficulty in hemostasis at the operation. Her recovery was, contrary to all expectations, satisfactory without hemorrhagic diathesis, infection nor blastic crisis. Six months after operation, she is doing well and it is unnecessary to reinstitute chemotherapy for CNL. To our knowledge, this is the first case report of open-heart surgery in a leukemic patient in Japan.