Five hundred and forty-six patients in a consecutive series of 1,601 patients with craniocerebral trauma had computed tomography. One hundred and ninety-six patients had a follow-up CT scan. Thirteen patients (6.6%) developed apparently "silent" subdural hygromas of delayed evolution noted from six to 46 days after injury (average 22 days). Three of 10 patients (30%) improved after operation. No patient with a severe cerebral deficit (decortication or decerebration) improved. The three unoperated hygromas and the six that persisted after operation tended to resolve spontaneously. The infrequent and modest improvement following surgical treatment and the tendency to spontaneous resolution suggest that operation may be unnecessary in many patients with posttraumatic subdural hygroma of delayed evolution.