We reviewed serial computed tomographic (CT) scans of 58 patients with traumatic subdural hygroma (SDG) to investigate its natural history. All were re-evaluated with a special reference to the size and density of SDG. Thirty-four patients (58.6%) were managed conservatively and 24 patients (41.4%) underwent surgery. The lesion was described as remained, reduced, resolved, enlarged and changed. Means of interval from injury to diagnosis and any changes in CT were calculated. SDGs were resolved in 12 (20.7%), reduced in 15 (25.9%), remained in 10 (17.2%), enlarged in 2 (3.4%), and changed into chronic subdural hematoma (CSDH) in 19 patients (32.8%). SDG was diagnosed at 11.6 days after the injury. It was enlarged at 25.5 days, remained at 46.0 days, reduced at 59.3 days, resolved at 107.5 days, and changed into CSDH at 101.5 days in average. SDGs were developed as delayed lesions, and changed sequentially. They enlarged for a while, then reduced in size. The final path of a SDG was either resolution or CSDH formation. Nearly half of SDGs was resolved or reduced within three months, however, 61.3% of unresolved or unreduced SDG became iso- or hyperdense CSDH. These results suggest that the unresolved SDG is the precursor of CSDH.