A device for hyperthermia treatment in a body cavity or duct which has a narrow access pathway includes an elongate distal section for insertion into the body cavity comprising a centrally-located heat-releasing element enclosed by an elastic enclosure. The heat-releasing element is either surrounded by an elongate housing or is shaped to form a housing. An axially opening inlet passage opens to the proximal part of the housing to supply a heat-transmitting fluid to the housing. The housing has an outlet port to allow fluid to flow from the housing to fill and expand the elastic enclosure to exert pressure on the walls of the body cavity and an inlet port which allows fluid to return into the housing, thus permitting circulation of the fluid during treatment which results in uniform heating of the fluid. The heat-releasing element is formed of an inherently heat self-regulating type material and is either a semi-conductor material of PTC-type having a Curie temperature or Trip point, or a ferromagnetic material having a Curie point in combination with a supply of energy based on magnetic induction.