The pressure in 47 arteries feeding 21 arteriovenous malformations (AVM's) was investigated during transarterial embolization using a Tracker-18 microcatheter. On average, systolic pressure increased by 22 mm Hg. In AVM's with single or few feeders, embolization was usually achieved well; in contrast, giant AVM's with multiple feeders and a large arteriovenous shunt were poorly embolized. However, large AVM's with well-demarcated components may be reduced by embolization to an appropriate size for surgery or stereotactic radiation therapy. It was found that the feeding artery pressure increased significantly more in well-embolized than in poorly embolized cases. Measurements of the feeding artery pressure clarified the hemodynamics of AVM's and facilitated more successful embolization.