We investigated the incidence and mechanism of pulmonary artery hypertension (PAH) in a consecutive series of patients with aortic stenosis who were undergoing diagnostic cardiac catheterization. Forty-five patients with severe aortic stenosis were divided into two groups: group 1 comprised 13 patients (29%) with PAH (pulmonary artery systolic pressure > 50 mm Hg); group 2 comprised 32 patients (71%) without PAH. Group 1 patients had a higher incidence of congestive heart failure, a lower left ventricular ejection fraction and cardiac index, and more mitral regurgitation as compared with group 2 patients. Of the 13 group 1 patients, 8 had a transpulmonary gradient (pulmonary artery mean pressure--pulmonary capillary wedge pressure) > or = 10 mm Hg, consistent with reactive PAH. We conclude that PAH frequently accompanies aortic stenosis and is often reactive.