['ST-segment elevation (STE) due to inferior STE myocardial infarction (STEMI) may be misdiagnosed as pericarditis. Conversely, this less life-threatening etiology of ST elevation may be confused for inferior STEMI. We sought to determine if the presence of any ST-segment depression in lead aVL would differentiate inferior STEMI from pericarditis.', 'Retrospective study of 3 populations. Cohort 1 included patients coded as inferior STEMI, cohort 2 included patients with a discharge diagnosis of pericarditis who presented with chest pain and at least 0.5 mm of ST elevation in at least 1 inferior lead. We analyzed the presenting electrocardiogram in both populations, with careful assessment of leads II, III, aVF, and aVL. In addition, we retrospectively studied a third cohort of patients with subtle inferior STEMI (<1-mm STE with occluded artery on catheterization) and assessed the sensitivity of ST depression in lead aVL for this group.', 'Of 154 inferior STEMI patients, 154 had some amount of ST depression in lead aVL (100%; confidence interval, 98%-100%). Of the 49 electrocardiograms in the pericarditis group, all 49 had some inferior STE but none had any ST-segment depression in lead aVL (specificity, 100%; confidence interval, 91%-100%). In the third cohort, there were 272 inferior MIs with coronary occlusion, of which 54 were "subtle." Of these, 49 had some ST depression in lead aVL.'] Does sT depression in lead aVL differentiate inferior ST-elevation myocardial infarction from pericarditis?