In clinical practice, we need to develop new tools to identify the residual cardiovascular risk after acute coronary syndrome (ACS). This study aimed to evaluate whether the monocyte to high-density lipoprotein cholesterol ratio (MHR) variation (ΔMHR) obtained during hospital admission (MHR1) and repeated in the first outpatient evaluation (MHR2) is a predictor of major adverse cardiovascular events (MACE) after ACS. One hundred ninety-one patients admitted for ACS were prospectively included. The ΔMHR was calculated by subtracting MHR1 from MHR2. Patients were followed for 166±38 days in which the occurrence of MACE was observed. The best cutoff for ΔMHR was zero (0), and individuals were divided into two groups: ΔMHR<0 (n=113) and ΔMHR≥0 (n=78). The presence of MACE was higher in the ΔMHR≥0 (22%) than in the ΔMHR<0 (7%), with a hazard ratio (HR) of 3.96 (95% confidence interval [CI]: 1.74-8.99; P=0.0004). After adjusting for confounders, ΔMHR≥0 remained an independent MACE predictor with an adjusted HR of 3.13 (95%CI: 1.35-7.26, P=0.008). In conclusion, our study showed that ΔMHR was an independent MACE predictor after ACS. Thus, ΔMHR is a potential marker of residual cardiovascular risk after ACS.