Emergency planning for a major accidental release of chlorine gas from industrial installations into the community is outlined for emergency services and hospitals. Realistic planning has been made possible with the advent of computer models for gas dispersion which may be used to estimate the numbers of deaths and casualties, according to their severity. For most purposes sufficient accuracy may be obtained by using a small number of computer analyses for the most serious reasonably foreseeable events under typical day and night weather conditions, and allowing for the emergency response to be scaled up or down according to the size ofan actual release. In highly populated areas triage should be preplanned to deal with a large number of victims; field stations will be needed for the treatment and observation of minor casualties. The management and treatment of casualties is summarised. The best protection against a gas cloud is afforded by buildings whose windows, doors, and ventilation systems have been closed. Hospitals in the vicinity of an installation should draw up plans to protect patients and staff. Coordination in a disaster will require toxicological and epidemiological expertise and hospital plans should allow for this.In Britain emergency planning for industrial disasters involving fires, explosions, or toxic releases has been given an impetus by the introduction of the Control of Major Accident Hazards (CIMAH) Regulations, 1984.' A key innovation of these regulations is the obligation placed on local authorities, at county level, to prepare emergency plans for communities around the so called top-tier sites; currently there are over 200 large inventory, top-tier sites involved. Hospital disaster plans are likely to require modification to deal with the potentially large number of casualties caused by a major chemical release into the community, and county emergency planners are legally obliged to consult health authorities when drawing up their offsite plans for these special installations. Chlorine is the commonest toxic gas for which planning has to be undertaken and the problems it presents to medical planners are typical of irritant gases in general-for instance, ammonia and sulphur dioxide. We provide *This paper represents the view of the authors and should not be regarded as a statement of HSE policy.