Chronic bronchitis is the most frequent late respiratory disease among Iranians exposed to mustard gas during the Iraq-Iran war. The aim of this study was to investigate efficacy of oral and intravenous corticosteroid therapy in improving lung function in mustard gas induced chronic bronchitis patients. 65 mustard gas-exposed chronic bronchitis patients, who were not responsive to standard treatments in exacerbation occasions, were randomly divided into two groups: an intravenous group (39 patients) receiving 500 mg intravenous methylprednisolone daily, and an oral group (26 patients) receiving 1 mg/kg oral prednisolone daily. Corticosteroid was tapered over the study period in both groups. Spirometery was performed on admission and on day 8 of therapy for assessment of effectiveness of therapy. There was significant improvement in spirometery indexes of both groups in approximately half of the patients over the study period. Furthermore, there was no difference between the pulse corticosteroid versus oral corticosteroid therapy in these patients. Since short-term corticosteroid therapy has a significant effect on lung function of almost fifty percent of patients with mustard gas-induced chronic bronchitis in exacerbation occasions, we suggest a short-term bolus steroid treatment to triage the patients into responders and non-responders for subsequent treatment.