Survivors of childhood cancer and hematopoietic stem cell transplantation are subject to a range of cardiac late effects including conduction abnormalities. We present, the case of a 12-year-old girl diagnosed with inappropriate sinus tachycardia after a history of multiply relapsed acute lymphoblastic leukemia and an exposure history including anthracycline-based chemotherapy followed by the total body irradiation in the context of a hematopoietic stem cell transplant. The clinical importance of this late cardiac effect along with the potential options for therapy is discussed.