The purpose of this study was to evaluate the changes in arterial wall morphology induced by coronary stent implantation and the influence of plaque morphology on stent expansion by intravascular ultrasound. Intravascular ultrasound imaging was performed in 25 lesions before and after Palmaz-Schatz stent implantation. In the 25 lesions with ultrasound images before and after stent deployment angiographic percent diameter stenosis decreased from 71% +/- 11% to 6% +/- 14%. By ultrasound there was a gain in luminal area from 2.0 mm2 +/- 1.5 mm2 to 6.6 mm2 +/- 2.1 mm2 owing to gain in external elastic membrane area of 2.5 mm2 +/- 1.7 mm2 and reduction of plaque area of 2.1 mm2 +/- 1.7 mm2. Calcified lesions (n = 8) showed significantly less relative luminal gain (218% +/- 128% vs. 421% +/- 276%, P = .01), and stent expansion was significantly less symmetric (minimal/maximal stent diameter 0.8 +/- 0.1 vs. 0.9 +/- 0.1, P = .002) as compared to non-calcified lesions (n = 17). The difference in lumen area within the stent between the previously stenotic area and the ends of the stent was significantly larger in calcified lesions as compared to non-calcified lesions (29 +/- 28% vs. 8 +/- 23%, P = .03). Both vessel stretch and plaque reduction contribute to the luminal gain after coronary stenting. Calcified lesions interfere with optimal stent expansion.