A 10-month-old infant with Down's syndrome developed cyanosis due to a right-to-left shunt through a ventricular septal defect during sedated sleep induced for echocardiography. During cardiac catheterization, arterial oxygen saturation decreased, and arterial carbon dioxide concentration increased. Various echocardiographic parameters during sedated and natural sleep were compared. It was considered that hypoventilation caused by the narrowed upper airway was induced by sedated sleep. We conclude that the influence of sedatives should be considered in the estimation of echocardiographic data if an infant with Down's syndrome has a ventricular septal defect with pulmonary hypertension.