The evolution of white blood cells after ST elevation myocardial infarction (STEMI) and their association with infarct size and major adverse cardiac events (MACE) remains unclear. Two hundred eleven patients underwent CMR after STEMI. Infarct mass (grams) was determined. Neutrophil, lymphocyte, and monocyte counts (×1,000 cells/ml) were measured upon arrival and at 12, 24, 48, 72, and 96 h. Patients with large infarctions (3rd tertile ≥ 28.5 g vs. 1st and 2nd tertiles < 28.5 g) showed a larger neutrophil count at 12 h (14.8 ± 4.8 vs. 11.4 ± 3.3, p < 0.0001) and an increased monocyte count (maximum at 24 h (0.65[0.50-0.91] vs. 0.55[0.42-0.71], p = 0.004)) but no difference in lymphocyte count. Neutrophil count at 12 h independently predicted large infarctions (OR 1.14, 95%CI [1.04-1.26], p = 0.008). During follow-up (median 504 days), 25 MACE occurred. Neutrophil count at 96 h independently predicted MACE (HR 1.2, 95%CI [1.1-1.4], p = 0.003). Large infarctions show a marked neutrophil peak and an increasing monocyte count. Neutrophil count independently predicts large infarctions and MACE.