The aim of this study was to use myocardial contrast echocardiography to evaluate the effect of intravenous adenosine on microvascular reflow in patients with acute myocardial infarction who underwent primary coronary stenting (PCS). Thirty patients who underwent primary PCS for acute myocardial infarction were randomized to intravenous adenosine (50 to 70 mug/kg/min) or vehicle for 3 hours. Myocardial contrast echocardiography was performed before and sequentially after PCS to determine the risk area during coronary occlusion and infarct size. The risk area was similar in the adenosine- and placebo-treated patients. The infarct size as a ratio to the risk area was smaller in patients treated with adenosine when measured at 3 to 5 days (0.37 +/- 0.29 vs 0.68 +/- 0.25, p <0.01) and at 4 weeks (0.34 +/- 0.26 vs 0.60 +/- 0.21, p <0.01) after PCS. This effect was greatest when patency was achieved <4 hours after symptom onset (0.18 +/- 0.18 vs 0.74 +/- 0.31, p <0.05), with little effect after 4 hours. The relative microvascular blood volume in the risk area at 4 weeks was higher in patients receiving adenosine than in those receiving placebo (0.73 +/- 0.22 vs 0.57 +/- 0.20, p <0.01), and was highest when patency was achieved in <4 hours. In conclusion, the adjunctive use of intravenous adenosine after PCS reduces the infarct size relative to the risk area. This beneficial effect occurs primarily in those undergoing early intervention.