The airway response to the inhalation of ultrasonically nebulised distilled water was determined in 55 asthmatic patients and 16 normal subjects. We calculated the dose of water required to induce a 20% reduction (PD20) in forced expiratory volume in one second (FEV1) by measuring the output of the nebuliser and the volume ventilated by each subject. Forty-eight of the asthmatic patients had a PD20 of 9 ml or less but three patients required as much as 24 ml. A PD20 was not recorded in the normal subjects and the challenge was stopped after 33 ml. In 12 patients the challenge was repeated within six months and the airway response was shown to be reproducible at equivalent doses of water. In a separate group of 11 patients there was, however, a highly significant reduction in the percentage fall in FEV1 when equivalent doses of water were given on two occasions 40 minutes apart. When the temperature of the inhaled water was increased from 22 degrees C to 36 degrees C eight of 10 patients had a similar change in FEV1 with equivalent doses of water. The airways obstruction induced by the inhalation of water was readily reversed with salbutamol administered by aerosol. In some patients a challenge with water or 3.6% saline was repeated after pretreatment with sodium cromoglycate, atropine methonitrate, and verapamil hydrochloride, all given as aerosols. The airway response to the equivalent dose of water or saline was significantly reduced after treatment with sodium cromoglycate but not atropine or verapamil.