Myocardial calcification, a rare disease that leads to chronic or acute heart failure and with a poor prognosis, occurs in patients with abnormal calcium-phosphorus metabolism. The association between myocardial calcification and tumor lysis syndrome has not been reported to date. A 50-year-old man with hyperthermia and general malaise presented to our hospital and was clinically diagnosed with B-lymphoblastic leukemia (B-ALL) and febrile neutropenia accompanied by septic shock. Prednisolone was administered for tumor reduction. Two to three hours later, electrocardiography demonstrated ST elevation in V4-6, and blood tests showed elevated levels of cardiac enzymes. Transthoracic echocardiogram revealed diffuse severe hypokinesis with decreased left ventricular ejection fraction. Additionally, blood tests showed that serum phosphorus level increased to 8.0 mg/dl, which was likely due to tumor lysis syndrome. Circulatory and respiratory failure due to left heart failure progressed, and he died 3 days after administration of prednisolone. Pathological autopsy revealed diffuse proliferation of atypical B-lymphoblasts in the bone marrow, which led to the pathological diagnosis of B-ALL, accompanied by necrosis. On the cut surface of the heart, the left ventricle was dilated, and patchy yellowish-brown areas were present in the epicardial-side of the myocardium and spread through the circumferential wall of the left ventricle and interventricular septum. Microscopically, myocardial fibers were granularly basophilic in that area and were revealed as calcium deposits by Von Kossa staining. He was diagnosed with myocardial calcification. The drastic increase in the serum phosphorus level caused by tumor lysis syndrome seemed to be associated with myocardial calcification.