Abstract: A device and method for treating tissue inside a patient's body, the device includes an endoscopically introducible catheter shaft. An expandable chamber is mounted on the distal end of the catheter shaft. The chamber is defined by a flexible non-elastomeric wall. The chamber is associated with a first lumen defined by the catheter for fluid flow between the chamber and a fluid source outside of the patient's body. The chamber is filled with fluid after placement in the patient's body. When the expandable chamber is filled with fluid it has a diameter greater than the diameter of the transverse cross-section of the endoscope channel. According to the method, the endoscope is inserted into a patient's body and is used to view the inside of the patient's body, to determine the location of tissue to be treated. The catheter is inserted into the channel that passes through the endoscope. The wall of the expandable chamber is covered with electrodes connectable to an external radiofrequency electrical potential.