An 11-month-old boy with multiple congenital anomalies developed West syndrome and ACTH therapy was started. Marked bradycardia during sleep was observed after the 16th day of ACTH therapy. Echocardiography revealed both intraventricular septum and left ventricular free wall thickening with preservation of biventricular function. Both the patient's marked sinus dysfunction and his cardiac hypertrophy were suspected to be related to the ACTH therapy. Sinus function gradually improved after ACTH therapy was withdrawn and treatment with oral beta-agonist was started. We believe that the patient's sinus dysfunction and cardiac hypertrophy were caused by ACTH treatment because of the subacute nature of the onset and the absence of other potentially contributory factors such as infection or respiratory failure. Pediatricians should be aware that cardiac dysfunction could be associated with ACTH therapy for West syndrome.