Positive remodeling is more often observed in lesions of patients who have acute coronary syndromes or vulnerable (rupture-prone) plaques. However, there are few data that correlate plaque morphology, composition, and arterial remodeling in vivo. We evaluated coronary plaque characterization of lesions with positive remodeling using intravascular ultrasound (IVUS) radiofrequency data analysis. Seventy-seven nonbifurcation native coronary lesions (in 50 patients) were imaged in vivo using 30-MHz IVUS transducers. Lesions were classified into 4 plaque types, fibrous, fibrofatty, dense calcium, and necrotic core, by using processing of the radiofrequency signal validated in vitro. The remodeling index was calculated as the lesion external elastic membrane area divided by the proximal reference external elastic membrane area. Lesions were divided into 2 groups: positive remodeling (remodeling index>1.0, 26 lesions) and intermediate/negative remodeling (remodeling index<or=1.0, 51 lesions). Total plaque volume and the volume of each plaque type were averaged over the length of the lesion. Reference segment plaque compositions were similar. Mean lesion fibrofatty plaque area was significantly larger in lesions with positive remodeling than in lesions with intermediate/negative remodeling (1.2+/-0.7 vs 0.8+/-0.4 mm2, p=0.001; 26.3+/-6.6% vs 19.8+/-5.7%, p<0.001, of total plaque volume). The same results were obtained at the minimum lumen site and in the subgroup of patients who had acute coronary syndromes. Further, there was a linear relation between remodeling index and fibrofatty plaque area (r=0.26, p=0.02). In conclusion, in vivo IVUS radiofrequency data analysis demonstrates that positive remodeling occurs in lesions with more fibrofatty plaque.