Insertion of indwelling arterial catheter has become a valuable procedure for blood pressure monitoring and for measurement of blood gas tension during anesthesia. A 34-year-old man with chronic alcoholism and diabetes mellitus was admitted to the hospital because of hyperosm-olar coma. On the first hospital day, percutaneous left radial artery cannulation was carried out for hemodynamic monitoring and arterial blood gas analysis after a modified Allen's test appear-ed to be positive. On the eleventh hospital day, the catheter was removed because of fever and sign of infection at the insertion site. Treatment with vancomycin by sensitivity test was begun on day 13, after continued for 2 weeks. On day 34 a pulsatile mass was noted at the insertion site. During surgery, a pseudoaneurysm of 1X1.2 cm size was found, and liquified hematoma and necrotic material were noted. The aneurysm was removed and end to end anastomosis was successfully completed. We suggest that risk factors for pseudoaneurysm may include abnormal state of vessel wall, multiple attempts at cannulation, increased duration of catheterization, adva-nced age, prolonged hospitalization, hematoma and infection at cannulation site.