RF catheter ablation of ventricular tachycardia is sometimes limited by inadequate lesion depth. This study investigated the use of a retractable needle-tipped catheter to create deep RF lesions in vivo in porcine myocardium. An 8 Fr electrode catheter with an extendable 27-gauge needle at the tip was modified for RF ablation by embedding a thermocouple and attaching a pin connector. In three swine (32-58 kg) the left ventricle was entered via the femoral artery and endocardial contact was made. The needle was advanced 10 mm and 13 RF applications were made under a controlled temperature (90 degrees C x 120 s). Nine control lesions were made using a standard 4-mm tip catheter (60 degrees C x 120 s). The lesions were fixed, serially sectioned from the endocardium, digitally imaged, and quantified. Needle ablation lesions were deeper (10.15 +/- 0.77 vs 5.67 +/- 0.37 mm, P < 0.001) and more likely to be transmural (77 vs 11%, P = 0.008) than control lesions. The volume of control lesions, however, was larger (358.4 +/- 56.2 vs 174.7 +/- 18.6 mm(3), P = 0.002) due to a significantly larger cross-sectional area at the endocardium (0.548 +/- 0.04 vs 0.151 +/- 0.01 cm(2), P < 0.001). At depths > 6 mm, the needle electrode lesions had a greater cross-sectional area (0.136 +/- 0.01 vs 0.005 +/- 0.004 cm(2), P < 0.001). Catheter-based needle ablation is feasible and allows creation of deeper lesions that can be transmural. Although deep, the lesions had a small cross-sectional area such that precise targeting would be required for success.