BACKGROUND: The intracoronary stent implantation is regarded as an effective treatment modality of coronary artery disease. However, inadequate stent expansion was associated with subacute stent thrombosis and late restenosis. The high-pressure inflation stenting reduced the incidence of stent thrombosis and restenosis. Therefore, we intended to evaluate factors associated with suboptimal stent expansion after high-pressure balloon inflation. METHODS: One hundred and thirty eight patients with 150 lesions were treated with 168 stents of various types. Intravascular ultrasound(IVUS) images were obtained after angiographic optimization(<10% or residual stenosis) with high-pressure inflation stenting. In a subgroup of 61 selected patients with 61 lesion, IVUS images were obtained before predilation of the lesions. The quantitative off-line measurement of IVUS parameters were perfoemed. An optimal stent expansion was defined by IVUS as minimal stent cross sectional area(CSA) of > or =90% of the distal reference lumen CSA. Severe calcifications were defined by IVUS as are of calcium > or =180 degree. RESULTS: An optimal stent expansion was achieved in 67% of the 150 lesions. At preintervention, severe calcifications were in 10%(5/51) in optimal stent expansion and in 40%(4/10)in suboptimal stent expansion(p<0.05). At postintervention, minimal stent CSA was 8.0mm2 in optimal stnet expansion and 6.5mm2 in suboptimal stent expansion(p<0.01). Minor diameter at stented segment was 2.9mm in optimal stent expansion and 2.5mm in suboptimal stent expansion(p<0.01). The symmetrical index of stent expansion(SI, a ratio of the minor diameter divided by the major diameter) was 0.91 in optimal stent expansion and 0.85 in suboptimal stent expansion(p<0.01). The residual plaque burden at stented segment was larger in suboptimal stent expansion than in optimal stent expansion(56% vs 51%, p<0.01). CONCLUSIONS: Severe calcifications at preintervention, larger residual plaque burden and smaller SI at postintervention may be related to suboptimal stent expansion, as assessed by IVUS, despite angiographic optimization with high-pressure inflation stenting.