A 60-year-old man with Noonan's syndrome presented with generalized weakness, dyspnoea, and paresthesias. He was known to have pulmonic stenosis and atrial septal defect with right-to-left shunting and had refused therapy for many years. On presentation there was cyanosis and the haematocrit was 63%. After phlebotomy he was treated with balloon valvuloplasty. There was marked reduction in the transpulmonic gradient and oximetric evidence for reversal of his right-to-left atrial septal defect flow. After 18 months follow-up he remained system free and had no rise in haematocrit, consistent with no return of right-to-left shunting.