AV conduction disturbances were observed in 25.8% of all the patients with acute MI's, 48.8% of 43 inferior MI's and 4.5% of 44 anterior MI's. High degree AV block was observed in 20.0% of all the subjects, 39.3% of inferior MI patients, and none of anterior MI patients. The most severe AV blocks observed in each patients were 7(30.4%) first-degree, 5(21.7%) second-degree, and 11(47.8%) third-degree AV block. The initial AV conduction disturbances developed within 6 hours after onset of symptoms in 9(47.4%) and after 24-hours in 9(47.4%). Seven(30.4%) of 23 patients with AV block showed a transient progression in the degree of AV block, 5(29.4%) of 17 patients with first-or second-degree AV block progressed to third-degree AV block thereby constituting 45.5% of 11 third-degree AV blocks. Nine patients with early AV block less tended to progress in the degree of AV block than the patients with late AV block (1/9 vs 4.9). In early AV block the duration of high-degree AV block was shorter than late AV block(2.5 days vs 6.1 days). The duration of third-degree AV block was less than 2-hurs in 36.4~24 hours in 27.3%, and more than 24 hours in 36.4%. All third-degree AV blocks(90.9%) but one with the longest duration of 13 days returned to 1 : 1 AV conduction within 7 days. There was no significant difference in coronary angiographic findings including the incidence of stenotic lesion in proximal LAD and first septal perforator, number of involved vessel(s), and severity of RCA lesion between the patients with AV block and the patients without AV block in inferior MI.