The terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) assay identifies apoptosis and is used in transplant pathology to detect cardiac allograft rejection. We illustrate the use of TUNEL in identifying segments of ablated ventricular myocardium, and discuss its advantages over conventional histopathological stains. The TUNEL assay can be useful to investigators of catheter ablation therapy.