Radial artery cannulation has been established as a essential technique for direct arterial pressure monitoring and serial analyses of arterial blood gas, particularly in patients for open-heart surgery or critically ill patients. We performed a clinical, prospective study on radial artery cannulation in 49 patients who had received elective operation including open-heart surgery at Seoul National University Hospital from June, 1983 to July 1984. Allen's test was performed preoperatively to confirm the collateral circulation of the hand. In this study, incidence of thrombosis after decannulation was 46.9%. The correlation of sex, number of puncture, duration of cannulation, extracorporeal circulation and the use of inotropic agent to the incidence of thrombosis was sought. Among them, only extracorporeal circulation was significantly correlated to the incidence of thrombosis(p<0.05). Hematoma developed in 28.6% of the patients and neither infection of the cannulation site nor ischemia of the hand occured.