The purpose of the present study is to assess the ability of intravascular ultrasound to detect acute dynamic thrombus formation in canine blood vessels with damaged endothelium. Ultrasound catheters (20 MHz) were placed in the femoral arteries of anesthetized dogs, and imaging transducers were positioned at the sites of external constrictors applied to areas of endothelial injury. Flows were measured with externally applied Doppler crystals placed proximal to the constrictors. Twenty experimental procedures were performed in 18 dogs. Four procedures were performed using the InterTherapy system (4.9 Fr catheters) and 16 procedures were performed using the Boston Scientific/Diasonics system (6.0 and 4.8 Fr catheters). After injuring the endothelium by rubbing the adventitial surface with cushioned forceps, we placed the constrictors and catheters and found that femoral blood flow usually decreased to zero or near-zero over a 3- to 4-minute period. Striking the exposed artery dislodged the obstruction seen on the intravascular ultrasound images and restored flow to normal; spontaneous increases in flow associated with a reopening of the lumen were also noted. After femoral arterial blood flow returned to normal, another cycle of decreasing flow and thrombus formation was spontaneously initiated. Intravascular ultrasound images of thrombus formation were obtained in 18 of 20 experimental procedures, all associated with zero or near-zero arterial flow. Images obtained during spontaneous decreases in femoral artery flow demonstrated the gradual accumulation of material on the lumen. The obstructing thrombus had distinct borders and a "speckled" appearance on ultrasound, especially on dynamic images, which became increasingly bright and uniform with time. At the end of each procedure, the arterial segments were removed for histologic analysis. Gross thrombus was visible in all cases. Therefore, in this experimental model, intravascular ultrasound can successfully detect both the acute formation of thrombus associated with spontaneous episodes of decreased flow and the resolution of thrombus within injured and narrowed femoral arteries. Fresh thrombus has a unique ultrasound pattern that evolves gradually over time.