Costello syndrome is characterized by poor postnatal growth, mental retardation, curly hair, coarse face, loose skin of the hands and feet, and nasal papillomata. Patients with Costello syndrome have a high incidence of cardiac involvement, such as arrhythmias, hypertrophic cardiomyopathy, or congenital anomalies. The importance of cardiac involvement in Costello syndrome has not been strongly emphasized thus far, although arrhythmia and hypertrophic cardiomyopathy are both serious forms of cardiac involvement. We report the case of a Japanese girl with Costello syndrome, who experienced life-threatening cardiac involvement throughout her life.