The authors investigated the nature of a radiolucent line that was seen paralleling the radiopaque catheter wall on more than 70% of 176 pullout angiograms. Although this line may indicate a thrombus sleeve, accumulation of thrombus was rarely seen at the puncture site. A series of experiments demonstrated clearly that (a) an identical line could be reproduced in a bloodless model; (b) the width varied directly with the thickness of the catheter wall and appeared irregular in contour with 6.5, 7, and 8 French catheters: and (c) this line represented the outer third of the catheter wall.