From January 1987 to December 1989, we tried to treat conservatively for 44 patients with acute (within 24 hours) subdural hematoma at the Soonchunhyang University Chonan Hospital. The conservative treatment was performed for the patients who furfilled the following criteria ; 1) the thickness of the hematoma was smaller than the skull bone thickness(about 1cm), 2) the pupils were normal or rapidly became normal, and 3) there was little midline shift or little mass effect by the hematoma itself. The mean age was 36.8 years and the male to female ratio was 31 : 11. The mean interval from injury to CT scanning was 2.1 hours. All Patients were arrived within 5 hours after head injury. The mean GCS value on admission was 11.5. Abnormal pupils were found in 13 patients(29.5%). Most common cause of injury was road traffic accident. The mean thickness of the hematoma was 5.9mm and the mean volume of the hematoma was 12.6ml. The mean degree of midline shift was 2.5mm. Compression of the basal cistern, mesencephalic cistern and the lateral ventricle was observed in 20(45.5%), 17(38.6%), and 23 patients(52.3%), respectively. Skull vault fracture was present in 25 patients(56.8%). In the majority of the patients, they became alert within three days. Twenty-four patients(54.5%) were recovered without any deficits or sequelae. Only one patient(2.3%) required operative removal on the 9th hospital day(HD) due to enlarged hematoma and delayed recovery. Another patient required shunt operation due to communicating hyprocephalus 22 months later. Overall mortality rate was 25.0%. We discussed on the possible harmful effect of surgery and rationale of the conservative treatment for the thin ASDH.