IHACs (interhemispheric arachnoid cysts) are a rare kind of congenital arachnoid cyst that account for5% of all occurrences. The best surgical treatment for symptomatic IHAC is still debatable, and there are no clear standards. At the time of presentation, the median age was 13 months. Macrocrania with rapidly increasing head size, seizures, infantile spasms, and developmental delay were the most common clinical manifestations.Endoscopic cyst fenestration and shunt surgery are both safe treatments for IHAC. We present an 11-month-old male infant born to non consanguineous parents who has had a steady rise in head size since 6 months of age and delayed mile stones.A magnetic resonance image of the brain revealed a massive interhemispheric arachnoid cyst, as well as corpus callosum agenesis and hypoplasia in both frontal lobes. Baby underwent endoscopic cyst fenestration and cystoperitoneal shunt.