The worldwide epidemic of diabetes mellitus is increasing the burden of cardiovascular disease, the leading cause of death among persons with diabetes. The independent effect of diabetes on mortality following acute coronary syndromes (ACS) is uncertain. To evaluate the influence of diabetes on mortality following ACS using a large database spanning the full spectrum of ACS. A subgroup analysis of patients with diabetes enrolled in randomized clinical trials that evaluated ACS therapies. Patients with ACS in 11 independent Thrombolysis in Myocardial Infarction (TIMI) Study Group clinical trials from 1997 to 2006 were pooled, including 62,036 patients (46,577 with ST-segment elevation myocardial infarction [STEMI] and 15,459 with unstable angina/non-STEMI [UA/NSTEMI]), of whom 10 613 (17.1%) had diabetes. A multivariable model was constructed to adjust for baseline characteristics, aspects of ACS presentation, and treatments for the ACS event. Mortality at 30 days and 1 year following ACS among patients with diabetes vs patients without diabetes. Mortality at 30 days was significantly higher among patients with diabetes than without diabetes presenting with UA/NSTEMI (2.1% vs 1.1%, P < .001) and STEMI (8.5% vs 5.4%, P < .001). After adjusting for baseline characteristics and features and management of the ACS event, diabetes was independently associated with higher 30-day mortality after UA/NSTEMI (odds ratio [OR], 1.78; 95% confidence interval [CI], 1.24-2.56) or STEMI (OR, 1.40; 95% CI, 1.24-1.57). Diabetes at presentation with ACS was associated with significantly higher mortality 1 year after UA/NSTEMI (hazard ratio [HR], 1.65; 95% CI, 1.30-2.10) or STEMI (HR, 1.22; 95% CI, 1.08-1.38). By 1 year following ACS, patients with diabetes presenting with UA/NSTEMI had a risk of death that approached patients without diabetes presenting with STEMI (7.2% vs 8.1%). Despite modern therapies for ACS, diabetes confers a significant adverse prognosis, which highlights the importance of aggressive strategies to manage this high-risk population with unstable ischemic heart disease.