Background Urinary 11‐dehydro‐thromboxane ( TX )B 2 has been described as a potential predictive biomarker of major adverse cardiovascular events ( MACE s) in high cardiac risk patients. This part of LTIMI (Leukotrienes and Thromboxane In Myocardial Infarction) study aimed to evaluate the relationship between 11‐dehydro‐ TXB 2 and MACE s in patients with acute myocardial infarction ( AMI ). Methods and Results LTIMI was an observational, prospective study in 180 consecutive patients with AMI type 1 referred for primary percutaneous coronary intervention. On admission and at follow‐up visits (1 month, 1 year), 11‐dehydro‐ TXB 2 was measured in urinary samples by using high‐performance liquid chromatography–tandem mass spectrometry. The primary outcome was occurrence of composite MACE s during 1‐year after AMI . Left ventricular ejection fraction was assessed in echocardiography on admission and at 1‐year follow‐up. Analyses of 11‐dehydro‐ TXB 2 (pg/mg creatinine) were performed on log‐transformed data and expressed as median with IQR (Q1–Q3). 11‐Dehydro‐ TXB 2 level on admission was 7.39 (6.85–8.01) and decreased at 1 month (6.73, 6.27–7.12; P <0.001) and 1‐year follow‐up (6.37, 5.91–6.94; P <0.001). In univariate analysis, baseline 11‐dehydro‐ TXB 2 was higher in patients with MACE s (n=60; 7.73, 7.07–8.60) compared with those without MACE s (n=119; 7.28, 6.68–7.79; P =0.002). In multivariate regression model, 11‐dehydro‐ TXB 2 and 3 other variables (diabetes, multivessel disease, and left ventricular ejection fraction) were found to be best 1‐year cumulative MACE predictors with odds ratio for 11‐dehydro‐ TXB 2 of 1.58 (95% CI 1.095–2.33; P =0.017) and area under the curve (in receiver operating characteristic analysis of 0.8). Baseline 11‐dehydro‐ TXB 2 negatively correlated with both left ventricular ejection fraction on admission ( R =−0.21; P =0.006) and after 1 year ( R =−0.346; P <0.001). Conclusions 11‐Dehydro‐ TXB 2 predicts 1‐year cumulative MACE s in AMI patients and provides prognostic information on the left ventricular performance.