Eleven patients, three with acute anterior myocardial infarction and eight with anterior ischemia, who developed transient right axis deviation with a left posterior hemiblock pattern during the acute phase of myocardial infarction or ischemia are described (study group). A correlation between their electrocardiographic pattern and the angiographic findings was made. The arteriographie findings were compared with those of a group of 24 patients with acute anterior myocardial infarction or ischemia without transient right axis deviation (control group). The main electrocardiographic characteristics of the right axis deviation pattern were: 1) an average shift of the mean frontal axis to the right of 42° (10° to 94°); 2) increased voltage of R waves in leads II, HI and aVF and appearance of small Q waves or decreased voltage of Q waves if previously present in the same leads; 3) decreased voltage of R waves and appearance of deep S waves in lead aVL; and 4) inverted T waves and isoelectric ST segments in leads II, III and aVF. Coronary angiography revealed that the study group had a higher incidence of significant right coronary artery obstruction and collateral circulation between the left coronary system and the posterior descending artery than did the control group (100 versus 25% and 73 versus 0%, respectively; p < 0.01). There were no differences between the groups regarding left anterior descending and circumflex artery disease. Transient right axis deviation during anterior myocardial infarction or ischemia represents different degrees of left posterior hemiblock, which is probably a consequence of decreased blood supply to the left posterior fascicle and is highly correlated with the presence of significant right coronary artery stenosis and collateral circulation between the right and left coronary systems.