A method of ablating endometrial lining tissue, includes delivering radiofrequency power to the endometrial lining tissue of a uterine cavity, and modulating the delivered power so that a measured impedance of the endometrial lining tissue tracks a target impedance as a function of time, wherein the target tissue impedance is derived from a function that approximates a preferred endometrial lining tissue ablation impedance curve, scaled based upon a measured impedance of the endometrial lining tissue after RF power has been delivered for a predetermined initial time period.
