Introduction: Arachnoid cysts (AC) are benign pathological structures that are typically found during childhood, and represent approximately 1% of all intracranial lesions. Unlike cases of secondary cysts with a known causation, which can be attributed to inflammatory processes or traumatic brain injuries, the causation of the primary cysts is unknown. In the cases reported to date, the cysts present from the first imaging study conducted, which is why they are considered to be of a prenatal origin.
Case report: A boy who, at the age of 5 months, is found to have an AC of considerable dimensions that did not appear in a transfontanellar echographic study carried out for other reasons at the age of 2 months.
Conclusion: The importance of this case lies in the fact that it has been possible to show the appearance of a cyst after birth in a sequence of images in the case of a pathology which, according to all the theories and tests, is considered to have a congenital causation.