The origin of chronic subdural haematoma (CSDH) and the pathogenesis of subdural hygroma (SDG) are still controversial issues. These issues and relationships between these traumatic subdural lesions are discussed. The origin of CSDH is usually a SDG, although a few cases are caused by acute subdural haematomas (ASDH). Subdural hygroma is produced by separation of the dura-arachnoid interface, when there is sufficient subdural space. When the brain remains shrunken, the SDG remains unresolved. Any pathologic condition inducing cleavage of tissue within the dural border layer at the dura-arachnoid interface can induce proliferation of dural border cells with production of neomembrane. In-growth of new vessels will follow, especially along the outer membrane, then bleeding from these vessels occurs. These unresolved SDGs become CSDHs by repeated microhaemorrhage from the neomembrane. Although most victims with ASDH underwent surgery or died, some patients could be managed conservatively. Since the ASDH is usually absorbed within a few weeks, only a very few ASDHs become CSDHs, when there is a sufficient potential subdural space. Chronic subdural haematoma can arise from ASDH, but more commonly from SDG. Such transformation, or development of a new subdural lesion, is a function of the premorbid status and the dynamics of absorption and expansion.