In an attempt to evaluate the influence of inspiratory flow rate on the sensitivity of the bronchial challenge with ultrasonic mist of distilled water (UMDW), we studied 8 symptom-free asthmatic patients who inhaled 60 liters of UMDW at a flow rate of either 0.35 or 1.2 1/s on two separate occasions. Tidal volume and the total duration of the respiratory cycle were kept constant during all inhalation tests. The bronchial response was assessed in terms of percent changes of baseline airway conductance. The results demonstrate that the mean bronchial response after 0.35 1/s UMDW inhalation was significantly greater than after 1.2 1/s inhalation of comparable doses of UMDW. Furthermore, 3 patients who developed a significant bronchoconstriction after 0.35 1/s UMDW inhalation showed only a mild, not significant bronchial response when the mist was inhaled at 1.2 1/s. We conclude that the sensitivity of the UMDW challenge is increased if the agent is inhaled during slow, tidal breathing.