Seventeen patients with 91 pulmonary arteriovenous malformations were evaluated by superselective angiography. Seventy-two (79%) of the malformations consisted of a simple type characterized by a single feeding artery draining into a bulbous, nonseptated aneurysmal communication with a single draining vein. The other 19 (21%) were complex, consisting of two or more pulmonary artery branches communicating with a bulbous septated aneurysmal part with two or more draining veins. Two patients had diffuse pulmonary arteriovenous malformations confined to multiple lobes of the lung. Balloon embolotherapy was effective in permanently obliterating the malformations in 14 patients treated. Therapy provided a sustained rise in arterial PO2 in the sitting position from a mean value of 44 mm Hg before occlusion to 65 mm Hg after occlusion. In three patients the process was too diffuse and/or the malformations too large for therapy to be effective. Complex pulmonary arteriovenous malformations required occlusion of all feeding arteries. Balloon embolotherapy provided an effective treatment for pulmonary arteriovenous malformations regardless of anatomic type.