Objective: Microvascular obstruction (MO) is a factor of adverse outcome in patients with ST-elevated myocardial infarction (STEMI). We assessed the presence and extent of MO and its relationship with infarct size and left ventricular (LV) functional parameters after acute non-ST-elevated myocardial infarction (NSTEMI). Methods: Twenty-five patients with first acute NSTEMI underwent a cine and first-pass perfusion cardiac magnetic resonance (CMR) study, with late gadolinium enhancement imaging 72 h after myocardial infarction. Results: MO was detected in 32% of patients, and its extent comprised 0.5–3.1% of the total LV mass (mean 1.9 ± 1.2%). Patients with MO had a significantly larger infarct size than patients without (14.1 ± 5.9 vs. 5.3 ± 4.1% LV mass; p < 0.001). There was no significant difference between both groups for the LV functional parameters and LV ejection fraction (58.5 ± 6.8 vs. 62.6 ± 9.6%; p = 0.29). Patients with MO showed a higher troponin I release (570 ± 364 vs. 148 ± 103 IU; p = 0.003) and a higher creatine kinase release (29,887 ± 18,263 vs. 10,287 ± 5,283 IU; p = 0.007). Conclusions: In patients with acute NSTEMI, MO has a frequency similar to that observed in patients with STEMI and also correlates with the infarct extent. The prognostic significance on clinical outcome remains to be shown in this specific population.