Acute inferior myocardial infarction frequently involves the right ventricle. We hypothesized that right ventricular involvement, as diagnosed by ST-segment elevation in the right precordial lead V4R, may affect the prognosis of patients with inferior myocardial infarctions. In 200 consecutive patients admitted to the hospital with acute inferior myocardial infarctions, we assessed the prevalence and diagnostic accuracy of ST-segment elevation in lead V4R (as compared with four other diagnostic procedures) to identify right ventricular involvement and its prognostic implications for in-hospital and long-term outcomes. The in-hospital mortality after inferior myocardial infarction was 19 percent, and major complications occurred in 47 percent of the patients. The presence of ST-segment elevation in lead V4R in 107 patients (54 percent) was highly predictive of right ventricular infarction (sensitivity, 88 percent; specificity, 78 percent; diagnostic accuracy, 83 percent), as compared with the other diagnostic procedures. The patients with ST-segment elevation in lead V4R had a higher in-hospital mortality rate (31 percent vs. 6 percent, P < 0.001) and a higher incidence of major in-hospital complications (64 percent vs. 28 percent, P < 0.001) than did those without ST-elevation in V4R. Multiple logistic-regression analysis showed ST elevation in V4R to be independent of and superior to all other clinical variables available on admission for the prediction of in-hospital mortality (relative risk, 7.7; 95 percent confidence interval, 2.6 to 23) and major complications (relative risk, 4.7; 95 percent confidence interval, 2.4 to 9). The post-hospital course (follow-up, at least 1 year; mean follow-up, 37 months) was similar in patients with and in those without electrocardiographic evidence of right ventricular infarction. Right ventricular involvement during acute inferior myocardial infarction can be accurately diagnosed by the presence of ST-segment elevation in lead V4R, a finding that is a strong, independent predictor of major complications and in-hospital mortality. Electrocardiographic assessment of right ventricular infarction should be routinely performed in all patients with acute inferior myocardial infarctions.