Catheters used in clinical cerebral angiography were examined by scanning electron microscopy for buildup of thrombus. In 21 (67%) of 31 individuals studied with non-heparin-coated catheters, either cell aggregations or thrombi developed compared with seven (28%) of 25 individuals studied with heparin-coated catheters. The median size of the cell aggregations and thrombi on non-heparin- and heparin-coated catheters was significantly different (p less than 0.01). The slopes that estimated the probable rate of thrombus formation were also significantly different (p less than 0.01). This suggests the desirability of replacement of non-heparin-coated catheters if the angiographic procedure is extended.