We evaluated the presence of pulmonary artery diverticulum in patients with Williams syndrome in comparison with other conditions causing peripheral pulmonary artery stenosis (PPS). Angiographic characteristics of patients with a definitive diagnosis of Williams syndrome, by fluorescence in situ hybridization, between 1990 and 2008 were reviewed. These data were compared with those diagnosed with those for patients with PPS without Williams syndrome. Differentiating morphological features on angiography were compared between the groups, along with demographic and echocardiographic data. Twelve patients with a chromosomal diagnosis of Williams syndrome who underwent cardiac catheterization were identified. Seven were male. Eleven patients (91%) had supravalvar aortic stenosis and nine (81%) had PPS. Pulmonary valve stenosis was seen in two patients. Eight patients who were negative for Williams syndrome and had PPS were identified during the same period. Two had Alagille syndrome and one had Noonan syndrome. Mean age at catheterization was 5 years in the Williams group versus 8 years in the non-Williams group. Pulmonary artery diverticulum involving the main pulmonary artery was documented in all patients with Williams syndrome, while none of the patients in the other group had it. It originated at the bifurcation of the pulmonary artery in all. In conclusion, the angiographic appearance of a diverticulum as an extension of the main pulmonary artery is a consistent finding in patients with Williams syndrome. Compared to the classically described findings of supravalvar aortic stenosis or PPS, pulmonary artery diverticulum can be considered as a pathognomonic feature of Williams syndrome.