During radiofrequency ablation sufficient power must be delivered to achieve a proper lesion, while tissue temperatures are kept below 100 degrees C. Tissue temperature and hence lesion size are determined by electrode to tissue contact and convective cooling, which vary with any obtained electrode position in the beating heart. A tool for evaluation of the combined effect of these parameters prior to ablation to predict the resulting lesion size for any actually obtained electrode position would be valuable. A low power pulse prior to ablation will cause a small increase in tip temperature, which will reflect tissue contact and convective cooling of the tip for that particular electrode position and may thereby predict lesion dimensions after subsequent ablation at that site. Ablation was performed in vitro on strips of left ventricular porcine myocardium during different levels of convective cooling, variable contact pressure, and two electrode orientations in temperature-controlled and power-controlled modes. A 4-mm tip catheter was used. Prior to ablation a 30-second, 0.6-W test pulse was delivered and the increase in tip temperature (DeltaT) registered. The study found that DeltaT and lesion size were mainly affected by the external cooling of the electrode tip and less by the electrode to tissue contact. Increased cooling caused a lower DeltaT (P < 0.001), and the effect on lesion dimensions depended on the ablation mode. For temperature-controlled ablation a larger lesion due to increased power output was obtained (P < 0.0001) for increased cooling, whereas for power-controlled ablation a smaller lesion was obtained for increased cooling (P < 0.05). DeltaT can predict lesion size, but the interpretation of DeltaT depends on the ablation mode. For temperature-controlled ablation a high DeltaT indicates a small lesion and for power-controlled ablation a high DeltaT indicates a large lesion.