Following 333 radial artery cannulations, the frequency of complete occlusion of the vessel on the day after removal of the annula was 30%, decreasing to 24% by the 8th day. Arterial occlusion was assessed using a modified Allen's test and an ultrasonic flow meter. The frequency correlated with the type and size of cannula used, being least (3%) with 20-gauge Teflon cannulae, with the degree of bruising, and with the sex of the patient, being significantly greater in women (37.5%) than in men (27%). Other variables such as insertion technique, low cardiac output, anticoagulation after operation and the duration of cannulation were assessed. Using multivariate analysis of 13 variables, it did not appear possible to predict with any certainty the outcome of arterial cannulation. The frequency of major complications was small and no permanent sequelae were detected.