Obtaining blood samples from arteries is important in the diagnosis and the monitor-ing of treatment in many medical situations. While arterial puncture by needle is indicated for one or a low number of intermittent samplings, arterial catheterization is indicated for multiple sampling. Both procedures are subject to complications; those associated with catheterization include embolic phenomena, thrombus formation, arterial occlusion, aneurysm formation, arteriovenous fistula, infection, sensory loss, hematoma formation, bleeding, pain, inadvertent injection of drugs, and collection of inaccurate data. These complications may be precipitated by certain catheter mate-rials, by vessel and catheter size, by duration of cannulation, by irrigation, by pre-existing arterial disease, and by low cardiac output. Measures to prevent com-plications include choosing the most distal sites, using Allen's test or a modification thereof, using small diameter, short, Teflon catheters, securing catheters properly, using a continuous, slow infusion of heparinized saline, and not injecting potentially toxic or irritating substances through arterial catheters.