An operational definition of post-traumatic amnesia is presented and a standardized procedure for the measurement of post-traumatic amnesia, which has been clinically tested in over 100 patients with severe, closed head injury is described. Twenty patients with severe head injuries who were still experiencing post-traumatic amnesia (as defined in this study), were assessed on an independent test of learning ability, as were 20 other patients with severe head injury who were no longer suffering post-traumatic amnesia. The performance of the two groups differed significantly; those in a state of post-traumatic amnesia performed more poorly. Both these groups showed significant impairment when compared with a control group of 20 patients who were in hospital because of orthopaedic injuries that were suffered during a motor vehicle accident. These preliminary results are sufficiently encouraging to recommend this simple procedure for routine use in hospitals, to enhance the accuracy of measuring the severity of head injury. The medicolegal use of this measure is also discussed.