A rectal probe comprising a catheter (1) having a longitudinal axis around which is mounted an inflatable, low pressure, compliant balloon (5) having a temperature element associated with the wall thereof, means for holding the probe at a proper position within a patient's rectum and means for inflating the balloon when it is properly positioned to put the temperature element in intimate contact with the anterior side of the rectum. A method for monitoring a patient's rectal mucosal temperature during thermal treatment of the patient's prostate is also disclosed.