蜘蛛膜囊腫是先天性且通常無症狀的顱內病灶，自行消失的案例非常罕見，已知有此病灶的病人較一般人於輕微頭部創傷後引發後續的慢性硬腦膜下血腫，若造成腦部壓迫之症狀，臨床上通常會進行顱骨鑽孔引流手術，只引流血水並不會去特別處理囊腫，但在極少數情況下，囊腫會在術後追蹤時被發現慢慢消失。我們報告一例蜘蛛膜囊腫在接受硬性硬腦膜下出血手術後消退的案例，探討其可能的病生理機轉和相關文獻回顧。

Arachnoid cyst (AC) is usually asymptomatic, incidentally-found, and a well-known risk factor of developing chronic subdural hematoma (CSDH) after minor head injury in young patients. Spontaneous remission or rupture of arachnoid cyst is very rare. When chronic subdural hematoma occurs, burr hole drainage is indicated if mass effect presenting. Only the hematoma is drained, where the cyst is left untouched. Although the chance of an arachnoid cyst to disappear is low, here we presents a case where the AC disappears after burr hole operation. We subsequently discuss the possible pathophysiological mechanisms and the relevant literatures.