We conducted a prospective, controlled study to investigate the use of CK-MB concentration and newer methods such as troponin-T concentration and CK isoforms, in the assessment of myocardial damage caused by radiofrequency current or low energy DC catheter ablation. The study population consisted of 3 consecutive patients who underwent low energy catheter ablation, 28 consecutive patients subjected to radiofrequency ablation, and 4 patients who were subjected to radiofrequency energy ablation but also had external DC shocks for cardioversion of atrial fibrillation that occurred during the procedure. The control group comprised eight subjects undergoing electrophysiological study. Prior to ablation and at 30 minutes, 1, 2, 6, and 12 hours following the procedure, serial blood samples were taken for measurement of troponin-T and CK-MB concentrations, and calculation of the MM3/MM1 and MB2/MB1 ratios. Troponin-T concentration was raised above normal in all patients subjected to low energy ablation and in all but two patients subjected to radiofrequency ablation. Only 42% of all patients subjected to ablation had at least one raised CK-MB concentration postablation. The MB2/MB1 ratio was raised in all but had at least one raised CK-MB concentration postablation. The MB2/MB1 ratio was raised in all but two patients following radiofrequency or low energy ablation but it was also abnormal in the preablation samples in nine patients. The MM3/MM1 ratio failed to detect myocardial damage in 71% of all patients. There was a statistically significant difference between the control and patient groups regarding all four indices of myocardial damage. Low energy ablation caused a significantly higher degree of myocardial damage compared with radiofrequency (RF); this effect could not be attributed to different numbers of total energy discharges. Our results indicate that catheter ablation, regardless of the mode of energy used, inflicts detectable injury upon the myocardium. This injury can be quantitated by using newer analytical techniques, such as serial, postablation measurements of troponin-T concentration.