One hundred twenty-four follow-up scans were performed on 24 patients who had had posttraumatic subdural collections of low density. Of the 24 patients, 6 developed chronic subdural hematomas. Six other subdural collections showed a temporary increase in attenuation but eventually resolved. The remaining 12 subdural collections resolved without apparent increase in density. Illustrative cases are presented with computed tomographic scans. The identity of these posttraumatic subdural lesions of low density is discussed. They seem to be posttraumatic subdural hygromas.