A patient's body temperature is regulated by using an enclosure to enclose a portion of the patient's body in an isolated environment. In accordance with one embodiment, the patient's entire body below his head is enclosed. Once enclosed, the patient is surrounded by heated vapor that is injected into the enclosure. The heated vapor condenses on the patient's skin to warm the patient quickly, efficiently and safely. This embodiment is well suited for the treatment of hypothermia patients. In accordance with alternative embodiments, only a portion of the patient's head is enclosed. In these embodiments, an enclosure is placed on the patient's head, and heated vapor is fed into the enclosure. Excess air and vapor are preferably dissipated by vents in the cap, whereas excess condensate is preferably absorbed by an absorbent layer provided in the cap. These embodiments are well suited for regulating a patient's body temperature in a surgical setting. Tympanic temperature is monitored and thus vapor dew point temperature is controlled to less than 47° C. but greater than tympanic temperature. Heated vapor may heat other articles such as intravenous supply bags. The heated vapor for many locations in a building may be supplied from a central steam generator.