We report a 3-month-old girl with Costello syndrome complicating fatal hypertrophic obstructive cardiomyopathy. She had typical findings of this syndrome, slight dyspnea and persistent wheezing. Doppler echocardiography revealed asymmetric septal hypertrophy and systolic anterior movement of the anterior mitral leaflet. There was grade 1 mitral regurgitation. Although once her heart failure had been controlled medically, she died suddenly following deterioration of her heart condition. Costello syndrome can complicate fatal hypertrophic obstructive cardiomyopathy.