Five cases of nontraumatic intradiploic arachnoid cysts in elderly patients are reported. All cysts were located in the occipital bone and appeared as well-demarcated radiolucent lesions. The cysts were multiple in three cases. Presenting symptoms included headache or dizziness, but most lesions were asymptomatic and found incidentally. In the most recent three cases, magnetic resonance (MR) imaging revealed intradiploic cysts containing cerebrospinal fluid (CSF) with cerebellar herniation. Operation found the cysts filled with CSF and dural defects through which cerebellar tissue was herniating. In two patients, CSF leakage from the outer table occurred. Intradiploic arachnoid cyst seems to be congenital in origin but commonly found in the elderly. MR imaging is the most useful diagnostic method for differential diagnosis from other osteolytic skull lesions.