We report a case of Williams and Beuren syndrome in which the obstruction of the pulmonary vascular bed, linked with peripheral stenoses of the pulmonary arteries, associated here with an interventricular communication, was aggravated due to the coexistence of in situ thrombotic disease, leading to a progressive hypoxaemia, requiring long-term anticoagulant treatment, and compromising the long-term prognosis. This case report puts into question the so-called favourable prognosis of pulmonary arterial branch stenoses in this malformation syndrome.