The incidence of femoral artery spasm and subsequent thrombosis complicating angiography is greater in children than adults. This study was designed to assess femoral artery spasm and to determine its causes in children. In 100 unselected, consecutive infants and children undergoing left heart angiocardiography by the Seldinger technique, angiography of the femoral artery was performed to include the puncture site after completion of angiocardiography. Arterial spasm was quantified and, with mathematical analysis, related to various clinical and angiographic factors. Some degree of arterial spasm was noted in 62% of all cases; it was more frequent and severe in neonates. Although it was more intense in the infant with a small femoral artery, analysis of the data indicates that patient age, weight, length of catheterization, size of femoral artery, and catheter size have only minimal relative importance in the occurrence and severity of arterial spasm. The most important factor in spasm is the relative size of catheter to artery.