Background We examined the prevalence of undiagnosed diabetes or prediabetes and associations with ischemic outcomes among non–ST-segment elevation acute coronary syndrome (ACS) patients. Methods We categorized 8795 EARLY ACS trial patients into one of the following groups: “known diabetes” (n = 2860 32.5%; reported on the case report form), “undiagnosed diabetes” (n = 1069 12.2%; no diabetes history and fasting glucose ≥126 mg/dL or hemoglobin A1c ≥6.5%), “prediabetes” (n = 947 10.8%; fasting glucose ≥110 to <126 mg/dL, or “normal” (n = 3919 44.5%). Adjusted associations of known diabetes, undiagnosed diabetes, and prediabetes (versus normal) with 30-day and 1-year outcomes were determined. Results Undiagnosed diabetes was associated with greater 30-day death or myocardial infarction (MI) (ORadj 1.28, 95% CI 1.05-1.57), driven primarily by greater 30-day mortality (ORadj 1.65, 95% CI 1.09-2.48). Known diabetic patients had 30-day death or MI outcomes similar to those of normal patients, but 30-day mortality was higher (ORadj 1.40, 95% CI 1.01-1.93). Prediabetic patients had 30-day death or MI outcomes similar to those of normal patients. One-year mortality was greater among known diabetic patients (HRadj 1.38, 95% CI 1.13-1.67) but not among those with undiagnosed diabetes or prediabetes. Conclusions Undiagnosed diabetes and prediabetes were common among high-risk non–ST-segment elevation ACS patients. Routine screening for undiagnosed diabetes may be useful since these patients seem to have worse short-term outcomes and deserve consideration of alternative management strategies.