Features of coma during the first week after sever head injury were analysed in 700 patients. Coma is best defined as inability to obey commands, to speak, or to open the eyes. If eye opening is omitted from the definition then some less severly affected patients will be included in the early stages, the duration of coma will be overestimated, and in the later stages the distiction between coma and other unresponsive states may be blurred. Other features which correlate with responsiveness (as judged by motor response, speech, and eye opening) are pupil reactions and eye movements; respiratory abnormalities are less common and less closely related to other aspects of severity. A rigorous fefinition of coma is necessary for valid commparisons between individual patients and between different series of patients with head injury. This is essential for the assessment of alternative management regimens and for establishing predictive criteria.