Among 126 cases of chronic subdural hematoma, 6 cases (4.8%) were diagnosed as acute subdural hematoma in chronic stage. They were 3 males and 3 femals whose age ranged from 46 to 79 years (average 62.8 years). All of them received major trauma to the head, but surgical treatment was not necessary until a mean period of 22.7 days, ranging from 17 to 30 days. Acute subdural hematoma in its chronic stage has subdural granulation tissue similar to that of the outer membrane of chronic subdural hematoma. This membrane was formed in the dural surface of the hematoma and was partially liquefied near the arachnoidal surface, but none of them had a macroscopically well-formed inner membrane. The subdural granulation tissue was of irregular thickness ranging from 1-2 mm, and was tightly adhered to the hematoma. Histologically, the granulation tissue consisted mainly of old fibrous connective tissue, having less capillaries than the outer membrane, and with scant blood components and few inflammatory cells. Pigments such as hemosiderin or hematoidin, and macrophages laden with these pigments accumulated in the granulation tissue, while new hemorrhagic foci were rare around the capillaries. The border layer between the subdural granulation tissue and the hematoma was generally irregular and not clearly discriminated, where many fibroblasts with mitotic figures and the macrophages irregularly accumulated. Thus, acute subdural hematoma in chronic stage differed from chronic subdural hematoma, both in its clinical and pathological aspects. The results suggest that the existence of an inner membrane plays an important role in defining the chronicity of subdural hematomas. It is advisable to investigate not only the clinical course, but also the existence of an inner membrane and the histology of the subdural granulation tissue in order to differentiate acute subdural hematoma in chronic stage from chronic subdural hematoma.