Major depression disorder (MDD) significantly increases the risk for coronary heart disease (CHD) which is a leading cause of mortality in patients with MDD. Moreover, depression is frequently observed in a subset of patients following acute coronary syndrome (ACS) and increases risk for mortality. Here evidence implicating omega-3 (n-3) fatty acid deficiency in the pathoaetiology of CHD and MDD is reviewed, and the hypothesis that n-3 fatty acid deficiency is a preventable risk factor for CHD comorbidity in MDD patients is evaluated. This hypothesis is supported by cross-national and cross-sectional epidemiological surveys finding an inverse correlation between n-3 fatty acid status and prevalence rates of both CHD and MDD, prospective studies finding that lower dietary or membrane EPA+DHA levels increase risk for both MDD and CHD, case-control studies finding that the n-3 fatty acid status of MDD patients places them at high risk for emergent CHD morbidity and mortality, meta-analyses of controlled n-3 fatty acid intervention studies finding significant advantage over placebo for reducing depression symptom severity in MDD patients, and for secondary prevention of cardiac events in CHD patients, findings that n-3 fatty acid status is inversely correlated with other documented CHD risk factors, and patients diagnosed with MDD after ACS exhibit significantly lower n-3 fatty acid status compared with nondepressed ACS patients. This body of evidence provides strong support for future studies to evaluate the effects of increasing dietary n-3 fatty acid status on CHD comorbidity and mortality in MDD patients.