Before 1980, essentially no health-related effects had been observed for short-term (<1 h) exposures to sulfur dioxide (SO2) levels similar to those found in the ambient environment (≤1 ppm). In 1980 and 1981, the results from several studies indicated that asthmatics' airways were substantially more responsive to SO2 than those of nonasthmatic individuals and that, with moderate exercise, significant bronchoconstriction resulted from exposures to as low as 0.5 ppm SO2. Since then, a multitude of reports regarding short-term exposure of asthmatics to low SO2 levels have appeared in the literature. From these studies, reflex bronchoconstriction, mast cell degranulation, and other, as yet unidentified mechanisms have been implicated in the induction of this response. As for most gaseous pollutants, the exposure considerations involved in response induction include the concentration of the pollutant in the inspired air, the volume, rate, and mode of ventilation, and the duration of exposure. For inhalation of SO2 by asthmatics, the temperature/humidity of inspired air and the propinquity towards repeated exposures are also important influences on response. A broad range of responsiveness to SO2 exists within the asthmatic population. Precise knowledge of factors involved in this variable responsiveness is lacking; differences in nonspecific airway sensitivity and the severity of disease have been suggested. For exposures in which significant group mean increases in airway resistance were observed, substantial bronchoconstriction was also induced in many individual asthmatics. This bronchoconstriction was often associated with moderate to severe wheezing and respiratory distress and sometimes required termination of activity or medical intervention or both, that is, sufficient response to be of clinical relevance.