Backgrounds and objectives: Various methods are being used to induce maximal hyperemia for physiologic studies. However, the feasibility and hyperemic efficacy of the intracoronary(IC) continuous infusion of adenosine has not been defined well. We sought to evaluate the safety and efficacy of IC adenosine continuous infusion for the induction of maximal hyperemia for fractional flow reserve(FFR) measurement. Methods: Patients with angiographically intermediate lesions in a major epicardial coronary artery were prospectively and consecutively enrolled. Patients with myocardial infarction, unstable clinical conditions, regional wall motion abnormalities, reduced left ventricular systolic function, left ventricular hypertrophy, or arrhythmia were excluded. In phase I, FFR was measured after an IC adenosine bolus injection(40-80關g) and at three dosages of IC adenosine infusion(180, 240 and 300關g/min) in 30 patients. A phase II study was performed to compare the hyperemic efficacy of IC adenosine infusion(240關g/min) with IC bolus injection and intravenous(IV) infusion(140關g/kg/min) in 20 patients. Pressure measurements were performed using 0.014 inch pressure guide wires(Wavewire, Endosonics Inc.; PressureWire, Radi Medical Systems) Results: FFRs were measured in all except 2 patients that showed a complete AV block with IC adenosine infusion of 300關g/min. In phase I, 3 patients showed an AV block at an IC adenosine infusion of 300關g/min. No significant differences in FFR were observed at the 3 different doses of IC infusion(p=0.06). However, there was a trend toward a lower FFR after an infusion of 240關g/min(0.83짹0.07, p=0.09) or 300關g/min(0.83짹0.08, p=0.06) versus an infusion of 180關g/min(0.84짹0.08). In phase II study, FFR after an IC bolus injection(0.83짹0.06) was significantly higher than with IV(0.79짹0.07) and IC(0.78짹0.09) infusion(p<0.01). However, no difference in FFR was observed for IC and IV infusions. Time to maximal hyperemia and plateau time of IC adenosine infusion were 8.5짹3.0 and 21.1짹7.3 seconds, respectively. Conclusion: IC infusion of adenosine(240關g/min) seems to be a safe and effective method of inducing maximal hyperemia for FFR measurements.