Fractional flow reserve (FFR) is an index of the physiological significance of a coronary stenosis. Patients who have lesions with a FFR of > 0.80, even optimally treated with medication, have however a MACE rate ranging from 8 to 21%. Coronary plaques at high risk of rupture and clinical events can be also identified by virtual histology intravascular ultrasound (IVUS-VH) as plaques with high amount of necrotic core (NC) abutting the lumen. Aim of this exploratory study was to investigate whether the geometry and composition of lesions with FFR a parts per thousand currency sign 0.80 were different from their counterparts. Fifty-five consecutive patients in whom FFR was clinically indicated on a moderate angiographic lesion, received also an imaging investigation on the same lesion with IVUS-VH. Data on plaque geometry and composition was analyzed. Patients were subdivided in two groups according to the value of FFR (> or a parts per thousand currency sign0.80). Lesions with a FFR a parts per thousand currency sign 0.80 (n = 17) showed a slightly larger plaque burden than those with FFR > 0.80 (n = 38) (54.6 +/- A 0.7% vs. 51.7 +/- A 0.7% P = 0.1). In addition, they tend to have less content of necrotic core than their counterparts (14.2 +/- A 8% vs. 19.2 +/- A 10.2%, P = 0.08). No difference was found in the distribution of NC-rich plaques (fibroatheroma and thin-capped fibroatheroma) between groups (82% in FFR a parts per thousand currency sign 0.80 vs. 79% in FFR > 0.80, P = 0.5). Although FFR a parts per thousand currency sign 0.80 lesions have larger plaque size, they do not differ in composition from the ones with FFR > 0.80. Further exploration in a large prospective study is needed to study whether the lesions with FFR > 0.80 that are NC rich are the ones associated with the presence of clinical events at follow-up.