We assessed the association of BMI with all-cause and cardiovascular (CV) mortality in a contemporary acute coronary syndrome (ACS) cohort. Patients from the Australian Cooperative National Registry of Acute Coronary Care, Guideline Adherence and Clinical Events (CONCORDANCE) and Global Registry of Acute Coronary Events (GRACE Australia) between 2009-2019, were divided into BMI sub-groups (underweight: <18.5, healthy: 18.5-24.9, overweight: 25-29.9, obese: 30-39.9, extremely obese: >40). Logistic regression was used to determine the association between BMI group and outcomes of all cause and CV death in hospital, and at 6months. 8503 patients were identified, mean age 64±13, 72% male. The BMI breakdown was: underweight- 95, healthy- 2140, overweight- 3258, obese- 2653, extremely obese- 357. Obese patients were younger (66±12 vs 67±13), with more hypertension, diabetes and dyslipidemia vs healthy (all p<0.05). Obese had lower hospital mortality than healthy: all-cause: 1% vs 4%, aOR(95% CI): 0.49(0.27, 0.87); CV: 1% vs 3%, 0.51(0.27, 0.96). At 6-months underweight had higher mortality than healthy: all-cause: 11% vs 4%, 2.69(1.26, 5.76); CV: 7% vs 1%, 3.54(1.19, 10.54); while obese had lower mortality: all-cause: 1% vs 4%, 0.48(0.29, 0.77); CV: 0.4% vs 1%, 0.42(0.19, 0.93). When BMI was plotted as a continuous variable against outcome a U-shaped relationship was demonstrated, with highest event rates in the most obese (>60). In conclusion, BMI is associated with mortality following an ACS. Obese patients had the best outcomes, suggesting persistence of the obesity paradox. However, there was a threshold effect, and favourable outcomes did not extend to the most obese.