BACKGROUND: Generally, enlargement of arachnoid cysts (ACs) has been found mostly in cases occurring during early childhood. Therefore, progressively enlarged ACs found to be symptomatic in elderly patients are extremely rare, and the mechanisms have remained unexplored. CASE DESCRIPTION: Our first patient was a 72-year-old woman with memory disturbance, who had presented with a large cyst beneath the right temporal convexity 9 years previously. The annual follow-up magnetic resonance imaging (MRI) studies had revealed that the cyst had progressively enlarged. In addition, her memory disturbance had become advanced. Endoscopic cyst fenestration was performed between the cyst and lateral ventricle, resulting in a reduction of her symptoms. Our second patient was a 79-year-old woman with unsteadiness, who had presented with a large cyst under the right parietal convexity 6 years previously. The annual follow-up MRI studies had shown that the cyst had gradually enlarged. She subsequently developed left hemiparesis. Because the pyramidal tract was located between the cyst and ventricle, a cyst-ventricle shunt was placed to allow the cystic fluid into the lateral ventricle, with complete resolution of her symptoms. In both cases, MRI showed obliteration of the subdural spaces around the cysts. Endoscopic observations revealed that the arachnoid membrane was lined under the surrounding brain, leading to the diagnosis of an AC. CONCLUSION: The establishment of stable communication between a cyst and the normal cerebrospinal fluid space is important to treat symptomatic ACs characterized by progressive enlargement, even in elderly patients. The 1-way entry of the cerebrospinal fluid into the cyst and the closure of the surrounding subdural space might result in AC enlargement internally.