To identify the patient and dietary characteristics associated with low omega-3 levels in patients with acute myocardial infarction (AMI) and determine whether these characteristics are useful to identify patients who may benefit from omega-3 testing and treatment.Dietary habits of 1487 patients in the 24-center Translational Research Investigating Underlying disparities in acute Myocardial infarction Patients' Health status (TRIUMPH) registry between April 11, 2005, and September 28, 2007, were assessed by asking about the frequency of fast food and nonfried fish consumption. All patients had erythrocyte omega-3 index measured at the time of hospital admission for AMI. We used multivariable linear regression to identify independent correlates of the omega-3 index and modified Poisson regression to predict risk of a low omega-3 index (<4%).The proportion of patients with a low omega-3 index increased with more frequent fast food intake (18.9% for <1 time monthly, 28.6% for 1-3 times monthly, 28.8% for 1-2 times weekly, and 37.6% for ≥ 3 times weekly; P<.001). In contrast, a low omega-3 index was less common among patients with more frequent fish intake (35.1% for <1 time monthly, 24.9% for 1-3 times monthly, 16.1% for 1-2 times weekly, and 21.1% for ≥ 3 times weekly; P<.001). Fish intake, older age, race other than white, and omega-3 supplementation were independently associated with a higher omega-3 index, whereas frequent fast food intake, smoking, and diabetes mellitus were associated with a lower omega-3 index.Potentially modifiable factors, such as patient-reported fast food intake, fish intake, and smoking, are independently associated with the omega-3 index in patients with AMI. These characteristics may be useful to identify patients who would benefit most from omega-3 supplementation and lifestyle modification.