A selective antagonist of S2-serotonergic receptors, ketanserin, was administered intravenously during right heart catheterization to 14 patients with pulmonary hypertension complicating systemic sclerosis. A significant reduction in pulmonary vascular resistance was noted which was accompanied by an increase in cardiac output. Paradoxical increase of pulmonary artery pressure occurred in 3 patients, whereas 2 patients with mild pulmonary hypertension normalized both pulmonary pressure and vascular resistance. Our data support the hypothesis that serotonin, released during in vivo platelet activation, influences pulmonary vascular tone in systemic sclerosis.