BACKGROUND AND OBJECTIVE: Fractional flow reserve (FFR) has emerged as an easily obtainable, and accurate, lesion-specific parameters for the physiological evaluation of coronary artery stenosis. However, the effects of the lesion length and eccentricity on the FFR remain unclear. Therefore, the relationship between the lesion length and eccentricity to the FFR, and the lesion length to the degree of stenosis were studied to see if there was any significant influence on the trans-stenotic pressure gradients and the FFR. SUBJECTS AND METHODS: Intravascular ultrasound (IVUS) was performed and the FFR measured in 19 lesions, ranging from 80 to 90% in area stenosis (AST), using a pressure wire. The eccentric index at the most stenotic site, and the length of the coronary stenosis lesion, above 50 and 70% of the AST, at the most stenotic site were obtained using IVUS. The FFR was defined by the ratio of distal mean coronary pressure (Pd) to that of the aortic mean pressure (Pa) under hyperemia. RESULTS: The FFR showed no correlation with the eccentricity or the length of the stenosis lesions when they were more than 50% of the AST (p>0.05). However, the FFR showed a significant correlation with the length of the most stenotic lesions (r=-0.79, p<0.001) and the lesions with an AST of more than 70% (r=-0.47, p<0.05). The hyperemic pressure gradient across the lesion showed a significant correlation with the length of the lesion, but only to those with the most severe portion of stenosis (r=0.64, p=0.003). CONCLUSION: In the functional evaluation of coronary stenosis, the length of the most stenotic lesions, as well as the cross sectional area and minimal lumen area at the most stenotic lesion, should be considered, as this study has shown that the length of the most severe stenotic lesion could influence the FFR and trans-stenotic pressure gradients.