Background: Radiofrequency ablation using irrigated catheters is performed using a power-controlled mode. However, lesion size is dependent on current delivery at a particular impedance, such that a power value alone may not reflect actual energy delivery, resulting in lesion size variability at similar power settings. We hypothesized that modulating baseline impedance at fixed power settings affects ablation lesion dimensions. Methods: In 20 ex vivo swine hearts, radiofrequency ablation was performed using an irrigated catheter at a fixed power setting of 30 W per 20 seconds and a multistepped impedance load (100–210Ω). In 4 in vivo thigh muscle preparations and right atria, ablation was performed using similar power settings at 3 baseline impedances: low (90–130Ω), intermediate (131–180Ω), and high (181–224Ω). The relationship between baseline impedance, current, and lesion dimensions was examined. Results: Baseline impedance had a strong negative correlation with current squared ( I ² ) for all experimental models: ex vivo (R=−0.94; P <0.0001), thigh muscle (R=−0.93; P <0.0001), and right atria (R=−0.94; P <0.0001). Lesion dimensions at similar power settings were highly variable and directly related to I ² (width [R=0.853], depth [R=0.814]). In the thigh muscle, lesion depth was 8.2±0.7, 6.5±0.8, and 4.2±0.5 mm for low, intermediate, and high impedance, respectively ( P <0.0001). In right atria lines, low baseline impedance resulted in wider lines (7.2±1.4 mm) relative to intermediate (5.8±1.8 mm) and high impedance (4.7±1.7 mm; P <0.0001). Conclusions: Radiofrequency ablation in a power control mode results in variable lesion dimensions that are partially related to differences in baseline impedance and current output. Ablation at a lower baseline impedance results in increased current output and lesion dimensions.