Brief episodes of myocardial ischaemia may enhance the tolerance to subsequent ischaemic episodes. This phenomenon has been called ischaemic preconditioning. Intracoronary electrocardiograms (ECG) were obtained during coronary angioplasty in 15 patients (13 males, 2 females; age 49 +/- 12 years) by attaching the external end of the balloon angioplasty guide wire to the V1 lead on the surface electrocardiogram. Surface leads I, II and III and intracoronary ECG were recorded at baseline and during balloon occlusion. The pattern of ST segment change during the first and subsequent inflation were compared. A significant reduction in ST segment deviation was observed in intracoronary ECG between the first and second inflation (10 +/- 1 vs 5 +/- 3 mm, p < 0.05). However, surface ECG showed no significant difference in ST segment deviation between the two inflations (3 +/- 1 vs 2 +/- 1 mm, p = NS). Thus, intracoronary ECG detects acute ischaemia more readily as compared to the routinely monitored surface ECG. A significant reduction noted in ST segment deviation during the second inflation suggests the phenomenon of ischaemic preconditioning in humans.