Recently, cryoablation has received increased attention as a safer alternative to radiofrequency (RF) ablation. The purpose of this study was to compare the effect of convective cooling at physiologic flow rates on RF lesion size and on cryo lesion size. Porcine hearts were sectioned into 40 pieces and placed in a temperature-controlled saline bath (37 degrees C) with varying directed flow rates (0, 1, 2 and 3 L/min). Large-tip cryoablation (8 mm tip) was performed for 4 minutes on 20 tissue sections at -80 degrees C tip temperature. Large-tip RF catheter ablation (10 mm tip) was performed at 60 degrees C target temperature for 1 minute on 20 tissue sections. For each catheter, flow rates were randomized between applications. The tissue pieces were placed in culture medium for 24 hrs, sectioned, stained and measured to determine lesion depth, width and volume. Average lesion geometry was estimated from the data. Lesion dimensions were dependent on the flow rate for RF ablation with larger lesions at higher flow rates (mean volumes: 211+/-35, 304+/-79, 439+/-125 and 525.7+/-187 mm3 for 0, 1, 2 and 3 L/min flow rate, respectively, p<0.01). Also for cryoablation lesion size varied significantly with flow, such that lower flow rates produced larger lesions (mean volumes: 855+/-402, 809+/-218, 658+/-91 and 360+/-14 mm3 for 0, 1, 2 and 3 L/min flow rate respectively, p<0.01). While RF ablation creates larger lesions at high flow rates (3 L/min), cryoablation creates larger lesions at low flow rates (0-1 L/min, p<0.05).