OBJECTIVES: This study sought to assess whether intracoronary adenosine or nitroprusside following thrombus aspiration (TA) is superior to TA alone for the prevention of microvascular obstruction (MVO) in ST-segment elevation myocardial infarction (STEMI) patients undergoing percutaneous coronary intervention (PCI). BACKGROUND: MVO, due to its multifactorial pathogenesis, still occurs after TA in a sizeable portion of patients. METHODS: We performed a placebo-controlled, randomized, open-label, blind-examination, multicenter trial. A total of 240 STEMI patients with Thrombolysis In Myocardial Infarction (TIMI) flow grade 0/1 were randomly allocated 1:1:1 to receive adenosine (n = 80), nitroprusside (n = 80), or saline (n = 80) given distal to the occluded site after TA. The primary endpoint was the incidence of ST-segment resolution (STR) >70% on surface electrocardiogram at 90 min after PCI. Secondary endpoints were angiographic MVO incidence (TIMI flow grade ≤2 or 3 with a myocardial blush grade <2) and major adverse cardiac event (MACE) rate at 30 days as a composite of cardiac death, myocardial infarction, target lesion revascularization, and heart failure requiring hospitalization. RESULTS: STR >70% occurred in in 71% of adenosine-treated patients, in 54% of nitroprusside-treated patients, and in 51% of saline-treated patients (p = 0.009 and p = 0.75, respectively, vs. saline). Angiographic MVO occurred in 18% of adenosine-treated patients, in 24% of nitroprusside-treated patients, and in 30% of saline-treated patients (p = 0.06 and p = 0.37, respectively, vs. saline). MACE occurred in 10%, 14%, and 20% of patients, respectively (p = 0.08 and p = 0.29 vs. saline). CONCLUSIONS: In STEMI patients treated by PCI and TA, the additional intracoronary administration of adenosine, but not that of nitroprusside, results in a significant improvement of MVO, as assessed by STR.