A 26-year-old female patient applied to the outpatient clinic in 2018 due to pain that had been present for a long time (16 years) and had recently spread from the top of the head to the nape of the neck. It was learned from the patient's anamnesis that her pain was relieved with analgesics. Although no pathological findings were found in the neurological examination, brain MRI was requested to reveal possible intracranial pathological conditions. In the MRI images taken, a cystic mass with a lobulated contour and thin septa of 24x31x40 mm and similar intensity to CSF was detected in the right parasagittal area posteriorly at the vertex level. The patient's control brain MRI images 3 months later showed no increase in cyst size. However, the patient's anamnesis, which came for a control in 2022, showed that his current complaints and headaches had increased. The MR images showed that the existing cyst had reached a size of approximately 60x35 mm. Medical treatment was initially planned for the patient (Acetazolamide, 750 mg/day). The MR taken one month later showed that there was no reduction in the cyst size and the optic discs were slightly swollen in the fundus examination. Considering the cyst growth, the patient's failure to benefit from medical treatment and the swelling noted in the optic discs, the patient was fitted with a cystoperitoneal shunt. The patient's early postoperative tomography image showed a significant reduction in the cyst (50.15x25.66) and a slightly swollen optic disc image with blurred borders was detected in the fundus examination. The patient's complaints regressed, and in the radiological images it was determined that the cyst continued to shrink [(45.8x23.19 mm) and (41.1x20.3 mm)], the shunt was in place, and she had no clinical complaints. As a result, it was argued that patients with atypically located arachnoid cysts, as in this case, should be taken under close clinical and radiological follow-up, and surgical treatment methods should definitely be considered in patients who are found to have an increase in cyst size and clinical findings during this follow-up and who do not respond despite medical treatment.