BACKGROUND: Despite the 2000 and 2007 redefinition of myocardial infarction (MI), patients who are troponin (Tn) positive ([+]) but MB negative ([-]) may not be considered to have MI, particularly in the absence of known coronary disease (prior MI or revascularization; coronary artery disease [CAD]). How this affects treatment and outcomes has not been well described. METHODS: Direct arrival patients with non-ST elevation MI (NSTEMI) enrolled in the American College of Cardiology NCDR ACTION-GWTG Registry were included. Patients missing marker data who were Tn (-) and had CAD were excluded. Troponin (+) patients were categorized as MB (+) (n = 11,563) or MB (-) (n = 4,501). Treatments and in-hospital outcomes were compared between the 2 groups using logistic regression. RESULTS: Of the 16,064 NSTEMI patients, 28% were MB (-). The MB (-) patients were older (median age 68 vs 65 years) and had more comorbidities (hypertension 71% vs 66%, diabetes 31% vs 27%, heart failure 22% vs 19%; all Ps < .01). After adjusting for baseline characteristics, MB (-) patients were significantly less likely to receive clopidogrel, antithrombins, glycoprotein IIb/IIIa antagonists, or angiography (all Ps < .001). In-hospital mortality was lower in MB (-) patients (3.8% vs 4.9%, P < .01), which remained significant after adjusting for baseline variables (odds ratio 0, 69, 95% CI 0.6-0.9, P = .002). CONCLUSIONS: Patients without known CAD who have NSTEMI and are MB (-) have a higher risk profile but are less likely to receive guideline-recommended acute pharmacologic treatment than those who are MB (+). Given the relatively high mortality in this group, increased emphasis on improving quality of care in Tn (+)/MB (-) patients is warranted.