When inhaled, chlorine is a highly irritating gas that can damage larger airways as well as distal lung structure. We examined the short-term respiratory effects of acute chlorine inhalation during a swimming pool accident. On October 22, 1998, an emission of chlorine vapours occurred in the chlorinating maintenance procedures room of a recreational center in Rome. A total of 282 subjects (134 children) inhaled hydrogen chloride and sodium hypochlorite. Most people received bronchodilators and cortisone at the emergency room; five children were hospitalised. A telephone follow-up was taken on 260 (92.2%) subjects to obtain information about duration of exposure (less than 3 minutes, 3-5 minutes, more than 5 minutes), intensity of exposure (not at all/a little, a fair amount, a lot), and acute respiratory symptoms. A total of 236 (90.7% of the target) individuals underwent clinical examinations, which took place 15-30 days after the accident. Lung function was measured in 184 people (82 children) after 15-30 days. A multiple linear regression was run to test the association between chlorine exposure and lung function, separately in adults and in children. Acute respiratory symptoms occurred among 66.7% adults and 71.6% children. The incidence rates were highest among those who had chronic respiratory disease and had a longer duration of exposure. In about 30% of the subjects, respiratory symptoms persisted 15-30 days after the accident. Lung function levels were reduced among those who reported high intensity of exposure in comparison to those who reported low exposure, both in children and in adults (forced expiratory volume in one second, FEV1, ml differences: -109 (CI 95%: -310, 93) and -275 (CI 95%: -510, -40). Given high prevalence of potential exposed people, findings from this study should alert public health authority about the possible clinical consequences.