Our study objectives were to evaluate whether or not airborne water droplets in mist or fog affect the occurrence of nocturnal attacks of asthmatic children using a retrospective study. This study included 971 visits by children with bronchial asthma to the emergency department at nighttime (from 18:00 to 09:00) during a 3-year period (April 1, 1998-March 31, 2001). Meteorological data were checked at a local fire station and regional meteorological observatory. We divided nighttime into five 3-hour periods to evaluate the relationship between chronological changes in the frequency of the emergency department visits of asthmatic children and of meteorological conditions. In four of five periods of nighttime, multivariate analysis showed that mist or fog, average atmospheric temperature, and barometric pressure were related to the number of emergency department visits (n=1096, r=0.165-0.263, p<0.0001). We divided the year into four seasons to eliminate differences between atmospheric temperature and barometric pressure on clear nights and on misty or foggy nights; we also found the mean number of emergency department visits was higher on misty or foggy nights than on clear nights in each seasonal period (p<0.01). In addition, average atmospheric temperature on misty or foggy nights with the emergency department visits was higher than that on misty or foggy nights without any visits (p<0.01). Asthmatic children frequently visited the emergency department on misty or foggy nights, especially during midnight to dawn periods with high atmospheric temperature. Because a higher atmospheric temperature on misty or foggy nights indicates a larger saturated amount of airborne water droplets, our results suggest that mist and fog, in particular a saturated amount of airborne water droplets, may be a stimulus for bronchoconstriction.