The authors report a case of sarcoma of the pulmonary artery in a 41 year old woman hospitalised for syncope on effort associated with effort dyspnoea. Chest Xray and echocardiography were "normal", leading to referral for pulmonary scintigraphy and angiography which showed multiple intravascular filling defects in the pulmonary arterial tree. A tumour implanted on a pulmonary valve was discovered at exploratory thoractomy. It was resected and the pulmonary valve replaced with a Carpentier-Edwards bioprosthesis. The histological findings were those of a leiomyosarcoma. The patient was reoperated 19 months later for a local recurrence but the tumour was too extensive for a complete ablation. The patient developed right heart failure and died the day after surgery. Sarcoma of the pulmonary artery is a rare tumour in adults and carries a very poor prognosis. The clinical course is initially insiduous and polymorphic but early diagnosis and complete excision are the only means of improving survival.