Axial, dual spin-echo magnetic resonance (MR) images, taken at the level of the pulmonary arteries and gated to the cardiac cycle, were qualitatively and quantitatively evaluated in nine patients with primary pulmonary arterial hypertension and six controls. In controls and patients, intravascular signal intensity was higher during diastole than during systole when fast flow conditions exist in the arteries; however, patients with severe pulmonary arterial hypertension showed significantly higher signals in the pulmonary arteries than did controls. A correlation between pulmonary vascular resistance and the MR signal in the right pulmonary artery in early systole (r = 0.89) showed the ability of MR images to provide information on blood flow and suggests a role for magnetic resonance in assessing the severity of this disease noninvasively. Flow-related cardiosynchronous variations in the lung parenchyma of controls were also observed; with further development, magnetic resonance may become useful to measure tissue perfusion and provide both pathoanatomic and pathophysiologic information. Furthermore, there was a significant difference between the MR signal intensity of dorsal and ventral lung regions.