To determine whether precordial ST segment depression during acute inferior myocardial infarction indicates posterolateral wall ischemia, anatomical predominance of coronary circulation was examined by coronary angiography and evaluated in 43 patients who experienced first acute inferior myocardial infarction. Among patients who underwent intracoronary thrombolysis within six hours from the onset of symptoms, the infarct-related artery was the right coronary artery (RCA) in 35. In addition, their early 12-lead electrocardiographic features were compared with those in eight patients having the infarct-related left circumflex coronary artery (group Cx). Thirty-five patients with RCA obstruction were categorized in four groups: Four patients with left predominant type (group L), 10 with balanced type (group B), five with right super-predominant type (group SR), and 16 with right intermediate type (group RI). Seventeen of the 21 patients in groups SR and RI demonstrated precordial ST segment depression, whereas it was present in only six of the 14 patients in groups L and B (p less than 0.05). Of the 29 patients in groups SR, Cx and RI, total ST segment depression in leads V1 through V4 (sigma ST) was greater in the 14 patients in groups L and B (p less than 0.05) than in other groups. Furthermore, in these 29, all patients in groups SR and Cx had greater sigma ST than did the patients in group RI (p less than 0.05). There was no significant difference in sigma ST between groups SR and Cx. Precordial ST segment depression did not correlate with concomitant disease of the left anterior descending artery and was not a mirror image of ST segment elevation in inferior leads. On thallium-201 scintigraphy, additional perfusion defects of the posterolateral wall were present in all eight patients in group Cx and in ten of the 21 patients in groups SR and RI. Thus, precordial ST segment depression during acute inferior myocardial infarction seemed to be affected by the pattern of coronary circulation. It was concluded that this ST depression represents more extensive involvement of the posterolateral wall in patients with right predominant coronary circulation as well as in those with left circumflex artery obstruction.