Intracranial arachnoid cysts are developmental anomalies that are generally asymptomatic. We describe a 6-month-old boy with macrocephaly but normal neurological development who was found to have a rare, massive basal arachnoid cyst occupying most of the cranium and extending superiorly, causing significant shift due to mass effect. A cystoperitoneal shunt was placed, producing both a decrease in the arachnoid cyst dimensions and a concomitant reexpansion of parenchyma. After consideration of various management options, such a shunt system appears to offer a low risk of complications and a high likelihood of success.