Depending on stenosis severity, collateral flow can be a confounding factor in the determination of coronary hyperemic microvascular resistance (HMR). Under certain assumptions, the calculation of HMR can be corrected for collateral flow by incorporating the wedge pressure (P(w)) in the calculation. However, although P(w) > 25 mmHg is indicative of collateral flow, P(w) does in part also reflect myocardial wall stress neglected in the assumptions. Therefore, the aim of this study was to establish whether adjusting HMR by P(w) is pertinent for a diagnostically relevant range of stenosis severities as expressed by fractional flow reserve (FFR). Accordingly, intracoronary pressure and Doppler flow velocity were measured a total of 95 times in 29 patients distal to a coronary stenosis before and after stepwise percutaneous coronary intervention. HMR was calculated without (HMR) and with P(w)-based adjustment for collateral flow (HMR(C)). FFR ranged from 0.3 to 1. HMR varied between 1 and 5 and HMR(C) between 0.5 and 4.2 mmHg·cm(-1)·s. HMR was about 37% higher than HMR(C) for stenoses with FFR < 0.6, but for FFR > 0.8, the relative difference was reduced to 4.4 ± 3.4%. In the diagnostically relevant range of FFR between 0.6 and 0.8, this difference was 16.5 ± 10.4%. In conclusion, P(w)-based adjustment likely overestimates the effect of potential collateral flow and is not needed for the assessment of coronary HMR in the presence of a flow-limiting stenosis characterized by FFR between 0.6 and 0.8 or for nonsignificant lesions.