Background-Experience of balloon dilation of peripheral pulmonary stenosis (PPS) in Williams syndrome (WS) is limited. Methods and Results-Catheterizations in all patients with WS undergoing therapy for PPS from 1984 to 1999 were reviewed. Criteria for successful dilation included an increase Ͼ50% in predilation diameter and a decrease Ͼ20% in ratio of right ventricular (RV) to aortic (Ao) systolic pressure. Median age and weight were 1.5 years and 9.5 kg. There were 134 dilations during 39 procedures in 25 patients. The success rate for initial dilations was 51%. In multivariate analysis, successful dilation was more likely (1) in distal than in central pulmonary arteries (Pϭ0.02), (2)