The effects of cannulation of the radial artery were studied in 23 patients. Distal arterial pressure on the thumb, measured during radial or ulnar artery compression, was compared with the result of a modified Allen test performed before operation. Distal arterial pressure on the thumb was monitored for approximately 10 days after removal of the cannula. It was found that radial artery thrombosis, as defined by a thumb arterial pressure of less than 10 mm Hg during ulnar artery compression, occurred with a frequency of about 40% depending on the duration of cannulation. Furthermore, thrombosis of the radial artery resulted in a decrease in perfusion pressure in the thumb of from 17 to 33% of the values before cannulation. However, a satisfactory result from a carefully performed Allen test seems to ensure that post-thrombotic perfusion pressure in the thumb is not decreased to values associated with compromised nutrition of the tissues.