We present a series of 61 traumatic subdural hygromas, and discuss the pathogenic mechanisms and natural history of this condition. It commonly occurred in patients over 50 years of age and before 5 years of age. Fifty-two cases (85.2%) were diagnosed 3 days after head injury. Glasgow Coma Score (GCS) on admission was 3-8 in 24 (39.3%), 9-12 in 15 (24.6%) and 13-15 in 22 patients (36.1%). Although three patients (4.9%) showed slow deterioration, most hygromas were clinically 'silent'. Thirty-eight patients (62.3%) were managed conservatively and 23 patients (37.7%) underwent surgery. Only five patients (21.7%) showed gross improvement after surgery, even though surgery was performed only for enlarged hygromas shown by serial computed tomography. In five patients (8.2%), a chronic subdural haematoma subsequently developed from a hygroma. A favourable outcome (good recovery or moderate disability) occurred in 59%, an unfavourable outcome (severe disability and vegetative state) in 28%, and death resulted in 13%. Outcome was closely related to the severity of primary head injury.