Bronchial brushings to evaluate cytopathologic changes were collected from 28 patients exposed to a toxic atmospheric level (66 parts/million) of chlorine gas. Postexposure smears collected on day 5 showed basal-cell and goblet-cell hyperplasia, acute inflammation, and chromatolysis of columnar epithelial cells. Columnar epithelial cell syncytia were observed in 15 (53.57%) smears. Nine (32.14%) smears showed abundant nonpigmented alveolar macrophages. Seven (25%) smears from mucosal erosions showed proliferating fibroblasts and capillary fragments; on day 15 and day 25 repeat smears from these seven cases showed evidence of epithelial regeneration and repair by fibrosis. This latter feature could lead to chronic ventilatory incapacitation in patients exposed to toxic levels of chlorine gas.