The aim of the study was to evaluate by intracoronary ultrasound (ICUS) the efficacy of optimal coronary balloon angioplasty (POBA) guided by quantitative coronary angiography (QCA). The study population included 40 patients who underwent conventional coronary balloon angioplasty, in whom a stent-like result was achieved (percent diameter stenosis (%DS) < 35% in QCA). In all patients diagnostic ICUS assessment was performed after the procedure. The site of stenosis and the proximal and distal reference segments were analysed with respect to residual plaque burden (RPB), true vessel dimension (the media-to-media diameter) and type of vascular remodelling. Despite an optimal angiographic result residual plaque burden was 70 +/- 6% at the site of stenosis, whereas in the reference segments it was around 45% indicating the severity of atherosclerosis in angiographically normal vascular segments. No significant differences were seen between averaged reference vessel diameter in QCA (Ref.D) and averaged luminal reference diameter in ICUS. In contrast, true vessel diameter in ICUS was significantly larger than Ref.D in QCA (p < 0.001). Positive vascular remodelling at the site of stenosis was observed in most patients. Optimal angiographic result of QCA-guided POBA does not indicate optimal dilatation of the lesion. In most patients ICUS reveals marked residual plaque burden, which is an independent predictor of restenosis after percutaneous coronary interventions.