Nine cases with occlusion of the proximal portion of the subclavian artery are reported. Arterio-grams performed in 7 cases showed a reflex "siphoning effect" in the ipsilateral vertebral artery. Exercise of the affected upper extremity reproduced the symptoms in the first 6 patients. This phenomenon is considered to be caused by the siphoning of blood away from the basilar artery. One of the cases presented showed the "siphoning effect," but this patient''s symptoms could not be reproduced by exercise. However, he had complete abolition of attacks after "surgical bypass" of the subclavian occlusion. The last 2 cases presented also had occlusion of the subclavian artery without this phenomenon. Both of these patients had vertebral artery occlusion, in addition, and exhibited evidence of collateral circulation to the upper extremity through other cervical and thoracic channels.