A 32-year-old-female with single ventricle, 20 years after the Waterston operation is described. The right pulmonary artery was markedly enlarged and the blood flow across the Waterston shunt was 2 m/sec in systole and 1.5 m/sec in diastole. "Moyamoya" echo developed transiently in the markedly enlarged pulmonary artery when she suffered from bacterial bronchitis and PaO2 decreased. Continuous wave Doppler echocardiography showed decreased blood flow across the Waterston shunt. After the bronchitis was resolved and PaO2 returned to the basal level, "Moyamoya" echo was barely seen. This "Moyamoya" echo can be a reflection of decreased pulmonary blood flow, and was a useful marker in the follow-up of this patient.