The relationship between shift of the transitional zone on the standard 12-lead electrocardiogram (ECG) and anatomical rotation of the heart in one plane was studied by cardiac computed tomography (CT). Based on the position of the transitional zone, 102 subjects were divided into 3 groups: the normal transitional zone (NT) group (31 subjects), clockwise rotation (CWR) group (30 subjects), and counterclockwise rotation (CCWR) group (41 subjects). The left-sided angle between the interventricular septum and horizontal axis of the body (the septal angle) was determined on the cardiac CT and compared among the three groups. The angle was 51.8 degrees +/- 6.8 degrees in the NT group, 38.2 degrees +/- 8.4 degrees in the CWR group, and 64.9 degrees +/- 10.4 degrees in the CCWR group. There were significant differences in the septal angle among the groups (p less than 0.05). The mechanism of CWR and CCWR could be attributed to the septal angle in about two-thirds of the cases (24/30 in CWR and 27/41 in CCWR). However, those not explained by the septal angle amounted to 6 (20%) of the CWR group and 14 (34%) of the CCWR group. Relatively higher positions of the precordial ECG leads, as observed in the vertical heart, appeared to be responsible for CWR in the 6 patients, and left septal fascicular block was suspected to be responsible for CCWR in the 14 patients. These data indicate that about two-thirds of CWR and CCWR can be explained by anatomical rotation of the heart in one plane around the long axis, but other factors appear to be responsible for such electrocardiographic findings in the remaining one-third of cases.