Six consecutive patients whose clinical Allen test suggested a non-patent radial artery have been investigated by intra-arterial fluorescein angiography. Cannulation of the radial artery and injection of fluorescein typically perfused only the thumb and thenar eminence. Occlusion of the ulnar artery, leaving the radial artery as the dominant supply, resulted in fluorescein perfusion of the entire hand. It is concluded that the Allen test is of no clinical value.