Ablation catheters with multiple long coiled electrodes have been recently developed to induce continuous linear lesions for the treatment of atrial fibrillation. The efficacy and safety of ablation catheters with multiple long coiled electrodes has not been evaluated. The aim of the present in vivo study was to investigate the influence of saline irrigation on lesion dimensions and coagulum formation during RF current ablation using those ablation catheters. In 14 anesthetized sheep, the thigh muscle was prepared as a cradle and filled with heparinized blood (37 degrees C). The quadripolar coiled ablation catheter (electrode length 7 mm, electrode distance 2 mm) was placed parallel to the muscle with standardized 10-g contact pressure. RF current energy was delivered sequentially temperature-(70 degrees C) or power-controlled (10, 20, 30, or 40 W) with additional irrigation of the electrode (10 mL/min normal saline) for 90 seconds. Forty-two of 129 RF current lesions were induced by temperature-controlled and 87 by irrigated ablation. Except for three lesions following low energy irrigated application (10 W), all lesions were continuous. Significantly larger lesions following irrigated RF current applications were produced with a power output of 30 W (depth 0.74 +/- 0.13 cm, width 0.78 +/- 0.13 cm) and 40 W (depth 0.75 +/- 0.16 cm, width 0.92 +/- 0.28 cm) as compared to 20 W (depth 0.47 +/- 0.13 cm, width 0.82 +/- 0.22 cm). Coagulum formation adherent to the electrode was exclusively observed following 18 of 42 nonirrigated RF current ablations. In conclusion, irrigated coiled ablation electrodes induce continuous linear lesions with a power output of 20-40 W. The risk of coagulum formation at the coiled electrode can be avoided by irrigation.