The evaporative water loss from the respiratory tract was studied in 29 patients suffering from elevated body temperature due to disease. The method used was based on a fast acting aspiration psychrometer and an ultrasonic sensing flowmeter. A heating device was connected to the system so that no condensation could occur before the gases reached the site where the water vapour content and the minute volume were registered. It was shown that the minute volume was significantly increased when the patients temperature rose above 39 degrees C. The increase was 1.0 1.m-2 (25%). Consequently, the evaporative water loss from the respiratory tract rose 4.6 g.h-1 for a person with a body temperature above 39 degrees C and 1.75 m2 body surface. However, in clinical practice when replacing the increased water loss during high fever, the dermal loss which can amount to 500-1 000 g.24 h-1 due to sweating is of greater importance than the respiratory extra loss of about 110 g.24 h-1.