Simultaneous two-channel Holter monitoring, with a direct recording system, and maximal exercise testing with a 12-lead precordial electrocardiographic mapping system were performed in 50 patients with chest pain (41 with documented coronary artery disease, 9 without). The exploring Holter leads were placed to correspond to CM5 and an aVF-like lead. In 36 patients, ST segment changes were found with both Holter and the 12-lead precordial electrocardiogram, while in 12 patients no ischaemic changes were detected by either method. Thus the results of the two methods concurred in 48 of 50 patient (96%). The magnitude of the ischaemic change was similar in 24 of 36 patients (67%), while the Holter system underestimated the ischaemic change by 0.5-2.0 mm in 12 patients. When the maximal ST segment deviation in V5 was compared with CM5, the deviations with both systems were identical in all but one patient in whom a difference of 0.5 mm was found. The use of a Holter lead resembling a VF identified maximal ST segment change on only one occasion, and in only four patients was an ST segment change of 1 mm noted. In conclusion, ambulatory monitoring utilizing only CM5 seems to detect most episodes with ST segment changes, but the use of a 12-lead precordial mapping system during exercise testing may expand the possibility of defining the optimal sites for the exploring Holter leads to detect maximal ST segment change.