Twenty-three patients exposed to nitrogen dioxide in agriculture or industry were referred to the University of Wisconsin Medical Center. Eighteen experienced a transient upper respiratory tract syndrome; five developed pulmonary edema or bronchiolitis obliterans. This latter group responded to steroid therapy but all demonstrated evidence of persistent pulmonary dysfunction on follow-up studies. Combining our findings with those in the literature we concluded: (1) exposure to NO2 is more common than generally appreciated; (2) case fatality is high--29% for silo-filler's disease; (3) steroids are effective therapy and should be continued for at least eight weeks; (4) although the majority recover without significant sequelae, some individuals may develop persistent functional abnormalities; (5) there is no evidence that long-term exposure to low concentrations of NO2 leads to chronic airway obstruction; and, (6) NO2-induced pulmonary disease could be elminated with appropriate preventive measures.