BACKGROUND AND OBJECTIVES: Intravascular ultrasound (IVUS), following stent implantation, demonstrated a significant degree of underexpansion, despite the initial appearance of an angiographically successful deployment, in first-generation stents. With improvements in stent designs and delivery systems, the current-generation of stents appear to achieve optimal stent expansion. The purpose of this study was to evaluate optimal stent expansion, by nominal pressure balloon inflation, in the current generation of stents. SUBJECTS AND METHODS: We evaluated 30 patients having had Nir-Sox, Tristar, S670 or Bx Velocity stents successfully deployed at nominal pressure (7-10 atm) with delivery balloon system, between March and September 2001, using IVUS. IVUS criterion for optimal stent expansion was defined as a minimal stent area (MSA) ratio of > OR =0.8 of the average reference lumen area. RESULTS: The mean nominal balloon pressure was 8.87+/-0.9 atmospheres and the mean stent size was 3.38+/-0.45 mm. In reference segments, the minimal lumen diameter and average lumen area, found from the IVUS, were 3.18+/-0.51 mm and 8.88+/-2.92 mm 2 , respectively. In stented segments, the minimal stent diameter and MSA were 2.55+/-0.46 mm and 6.10+/-2.08 mm 2 , respectively. In only 11 of the 30 patients (36.7%) was the optimal stent expansion, by IVUS, achieved. CONCLUSION: Despite the development of a balloon delivery system for the current generation of stents, 63.3% of our study patients did not achieve optimal stent expansion, by IVUS, following nominal balloon inflation. Therefore, additional procedure will be required for optimal stent expansion in the current generation of stents.