The relationship between clinical features of brain dysfunction in the first week after severe head injury and outcome 6 months later has been analyzed for 1000 patients. Depth of coma, pupil reaction, eye movements, and motor response pattern, and patient age prove to be the most reliable predictors. The degree of brain dysfunction changes markedly soon after injury, and more reliable predictions of outcome result when assessment is based on the best level of functioning recorded in each early epoch. Predictions based on very early assessment are, therefore, often unduly pessimistic. Individual predictions of outcome, based on a large data bank, provide a powerful tool for assessing the relative efficacy of alternative treatments.