Exact determination of the percentage luminal stenosis after balloon angioplasty is essential when deciding to redilate or not, especially since the percentage luminal stenosis may be a predictor for long-term outcome. Conflicting percentage residual stenosis is frequently observed when angiography is compared with duplex or intravascular ultrasound measurements. The aim of the present study was to compare the percentage luminal stenosis after balloon angioplasty determined by duplex and intravascular ultrasound. In 22 patients, balloon angioplasty was performed in the superficial femoral artery to treat disabling claudication. Intravascular ultrasound studies were performed immediately after balloon angioplasty; duplex studies were performed 24-36 h after intervention. Intravascular ultrasound percentage luminal stenosis was calculated with respect to a proximal reference lumen. Duplex percentage luminal stenosis was determined by two methods: first, by assuming that the increase in peak flow velocity is directly related to lumen area; and second, by considering a peak flow velocity ratio of 1.6 and 2.4 is representative for > 30% and > 50% diameter stenosis, respectively. The percentage luminal stenosis calculated from duplex measurements was higher compared with intravascular ultrasound measurements (y = 0.38x + 20.1, r = 0.57). Excluding cross-sections with vascular wall damage (dissection or plaque fracture) over more than 60 degrees of the circumference improved the slope and correlation coefficient of intravascular ultrasound measurements versus duplex measurements (y = 0.88x + 7.8, r = 0.70). Thus, after balloon angioplasty, conflicting percentage luminal stenosis is frequently observed using intravascular ultrasound and duplex measurements. These differences in percentage luminal stenosis may partly be explained by the extent of vascular wall damage visualized on the intravascular ultrasound image.