There are some cases in which conservatively treated acute subdural hematoma (ASDH) does not disappear naturally and progresses to chronic subdural hematoma-like hematoma (CSDH) (hematoma with capsule formation). The objective of the present study was to identify factors which can be used to predict this unfavorable course during the early phase after the onset of the lesion. During the past 13 years, 10 of 96 cases of mild, conservatively treated ASDH (excluding suckling infants) progressed to CSDH, and those 10 patients showed the following background characteristics. There were 7 males and 3 females, and the mean age was 63.1 years. Five of the patients had a history of alcohol consumption, and one case each had a history of cerebral infarction, cerebral hemorrhage and a VP shunt. Acute-phase computerized tomography (CT) at the time of ASDH showed, in all 10 cases, an expansive-type lesion with a low density area in the hematoma, with expansion of the hematoma into the interhemispheric fissure. The hematoma was observed to undergo transient natural shrinkage in the acute phase. The period for progression to CSDH was indicated to be a mean of 20.5 days after the onset of the lesion, and its cure was possible with trepanation. In consideration of these results, it was surmised that ASDH patients with the following characteristics have a high risk of progression to CSDH during the subacute and chronic phases when conservative therapy is administered during the acute phase of the lesion: (1) old age, (2) a history indicative of brain atrophy, (3) an expansive-type image of ASDH on acute-phase CT, and (4) acute-phase CT indicative of cerebrospinal fluid mixing in the hematoma.