OBJECTIVE: Acute subdural hematoma (ASDH) with good initial Glasgow Coma Scale (GCS) score 13-15 is generally regarded as a mild head injury. However, the risk increases when significant amount of hematoma with midline shift exists. This study is to evaluate the clinical outcomes of patients with good neurological scores in spite of significant amounts of ASDH, and to compare the outcomes according to the treatment modalities. METHODS: Sixty patients with initial GCS score 13-15 in spite of significant amounts of ASDH and midline shifts were enrolled. They were divided into groups according to age, sex, side of location, initial GCS score, midline shift, and hematoma thickness. According to the therapeutic modalities, early craniotomy and initially conserved groups were identified, and initially conserved group was further classified into persistently conserved and delayed operation groups. The outcomes were measured by Glasgow Outcome Scale. RESULTS: Initial GCS score was a significant factor that influenced the final outcome (p=0.001). The outcomes were good in both early craniotomy and initially conserved groups without significant differences (p=0.268). Fifteen of initially conserved 49 patients underwent delayed operations from neurological deteriorations, but the outcomes were good without significant differences from persistently conserved group (p=0.481). CONCLUSION: Initial GCS score is an important factor that influences the clinical outcome. These patients can be conserved under close observations without early preventive craniotomies if no deteriorations are seen in the acute stage. Only those with delayed deteriorations may require simple operations such as burr hole trephinations which still guarantee good outcomes.