Fifteen patients were examined before and immediately after balloon angioplasty with a 4.3F, 30 MHz rotational tip intravascular ultrasound system. In 12 (80%) patients, 13 procedures could be analyzed because of sufficient image quality. The lesions for intravascular ultrasound (IVUS) study consisted of pulmonary arterial stenosis in 4 patients, Blalock-Taussig shunt stenosis in 3, coarctation of the aorta in 2, subclavian artery stenosis in 1, renal artery stenosis in 1, and ductus arteriosus in 1. Three distinctive morphologic types were identified: Group I had arterial stretching (3 patients); group IIa had superficial tearing (3); group IIb had deep intimal-medial tearing (5); group III had dissection (2). In the narrowest site, minimal and maximal luminal diameters, luminal area before and after balloon angioplasty were 3.5 +/- 1.8 mm vs 4.5 +/- 2.5 mm, 4.1 +/- 2.1 mm vs 5.4 +/- 3.5 mm, and 49.8 +/- 30.2 mm vs 88.3 +/- 45.2 mm2, respectively. The recoil value of group IIb with appropriate balloon angioplasty was approximately 0.3. IVUS may be an useful modality for evaluating the morphologic mechanism of dilatation after balloon angioplasty.