The frequency of complications following radial artery cannulation for monitoring purposes was determined in 1,699 cardiovascular surgical patients and in 83 patients in whom cannulation was performed in another artery after failure at the radial site. Patients were examined and radial artery flow determined by a Doppler technique 1 day and 7 days after decannulation. Although partial or complete radial artery occlusion after decannulation occurred in more than 25% of the patients, no ischemic damage to the hand or disability occurred in any patient. Neither duration of cannulation nor the size or material of the cannulas were determinants of abnormal flow. Abnormal flow was significantly related to female sex, the presence of hematoma, and to the use of extracorporeal circulation. The radial arteries of 16 patients whose results of Allen's test were abnormal were cannulated and no abnormal flow or ischemia followed. In 22 patients, the ulnar artery was cannulated after multiple punctures of the ipsilateral radial artery and no ischemia followed. We conclude that in the absence of peripheral vascular disease, the Allen's test is not a predictor of ischemia of the hand during or after radial artery cannulation, that when decreased or absent radial artery flow follows cannulation it is of no clinical consequence, and that radial artery cannulation is a low-risk high-benefit monitoring technique that deserves wide clinical use.