An apparatus (75) for treating a tissue lining of a mammalian body cavity, particularly a uterine endometrium comprising a pump (38) and a distendable bladder (10) connected to a catheter. The apparatus is introduced into the body cavity and the bladder is distended by introducing a non-toxic fluid under pressure. The fluid is heated by a heating element (20). A pump (38) circulates the fluid within the distendable bladder to help keep the fluid at a uniform temperature.