Fourteen cases, 5 with pre-existing COAD, exposed to up to 30 p.p.m. chlorine gas in an accidental leakage, were followed up clinically, radiologically and by spirometry at 2 weeks, 4 weeks, 8 weeks and 6 months. All the patients were asymptomatic by 2 weeks and did not reveal any radiological abnormality. The FVC, FEV1 and FVC observed/predicted improved at 4 weeks (p less than 0.05, p less than 0.05, p less than 0.01) and the improvement in FEF0.25-0.75 reached statistical significance (p less than 0.05) at 6 months. The mean improvement in FVC was 0.84 l and FEV1 was 0.6 l at 4 weeks. The 5 patients with pre-existing COAD did not show any evidence of additional lung damage. The observations have been consistent with acute tracheobronchitis with trends towards complete recovery.