Pulmonary thromboendarterectomy is now the treatment of choice for pulmonary hypertension due to chronic pulmonary thromboemboli. A precise assessment of location and extension of these thrombi is important because only proximal chronic pulmonary thromboemboli are accessible to surgery. Because intravascular ultrasound imaging can assess not only arterial luminal size, but also wall thickness, its value as a complement to angiography was assessed in 11 patients aged 35 to 64 years with severe pulmonary hypertension (systolic pulmonary artery pressure, mean +/- standard deviation 70 +/- 19 mm Hg; pulmonary artery resistance, 609 +/- 297 dynes.s.cm-5). Intravascular ultrasound was obtained in 10 of 11 patients and no complication occurred. Intravascular ultrasound identified 10 segments with suspected chronic pulmonary thromboemboli in 7 patients, all confirmed at operation. Nine segments were considered normal, all of which (except 1) were free of chronic pulmonary thromboemboli at operation. Image quality was highly dependent on pulmonary artery size and position of the catheter. Therefore, intravascular ultrasound of pulmonary arteries is feasible and safe in patients with pulmonary hypertension. It may help to assess the location and extension of the pathologic process involving pulmonary arteries.