Expiratory water loss was measured in four asthmatics and in four healthy subjects by leading the expired air through a tube filled with silicagel during hyperventilation. No difference in water loss was seen as the average water loss was 0.025 g/l for the asthmatics and 0.029 g/l for the healthy subjects. After hyperventilation PEF decreased more than 20% in the asthmatics while no change was seen in the lung function of the healthy subjects. It is concluded that expiratory water loss per se is not the reason for the decrease in lung function seen in asthmatics after exercise and hyperventilation.