Background The prognosis of acute inferior myocardial infarction is worse when it is complicated by right ventricular infarction. ST elevation in the right precordial leads is one of the reliable methods for detecting acute right ventricular infarction. The purpose of the study was to examine the relation between ST elevation in the right precordial electrocardiographic leads during acute inferior infarction and the severity of right ventricular systolic dysfunction. Methods This study analyzed the relation between ST elevation ≥0.1 mV in V₄R and the severity of right ventricular systolic dysfunction in 43 consecutive patients (men/women: 35/8; average age 62 ± 9 years) with acute inferior myocardial infarction with a rapid-response Swan-Ganz catheter to measure the right ventricular ejection fraction (RVEF). Results RVEF was significantly lower in patients with ST elevation (n = 18) than in those without (n = 25) (33% ± 6% vs 40% ± 9%, p = 0.010). If the infarct-related lesion was located in the proximal right coronary artery, RVEF tended to be lower than if the lesion was located in the distal right coronary artery or the left circumflex coronary artery (33% ± 10% vs 37% ± 9% vs 42% ± 9%, p = 0.101). Logistic regression analysis demonstrated that ST elevation in V₄R was the only independent predictor of depressed RVEF (odds ratio = 5.31, 95% confidence interval = 1.28 to 22.1, p = 0.022). Conclusion ST elevation in lead V₄R during acute inferior myocardial infarction predicts right ventricular systolic dysfunction. (Am Heart J 1998;135:689-95.)