Arachnoid cysts found under the age of 1 year are more likely to grow in size, relatively short term follow-up is required. Retrocerebellar location predicts a high risk of hydrocephalus, and the time window until irreversible neuronal damage is often narrow if the arachnoid cyst increases in size and becomes symptomatic. However, when and how to treat a neonatal or infantile patient with a retrocerebellar arachnoid cyst is still a controversial subject. We recently experienced 3 differently treated very young pediatric patients with retrocerebellar arachnoid cysts. One patient was treated two weeks after birth by a cystoperitoneal shunt. This patient showed normal development after the surgery. The other patient was treated by a ventriculoperitoneal shunt and subsequent cystoventriculostomy at the age of 4 months because of his mother's refusal on 14th day after birth. This one showed developmental delay despite of decreasing size of ventricles after the surgery. The last patient was treated with microscopic fenestration, which failed in its initial attempt. A revision operation by cyst excision succeeded and had no problem after the surgery. Therefore, we suggest that early surgical intervention for retrocerebellar arachnoid cyst can be considered. Although there are concerns of long term complications related to shunts, a cystoperitoneal shunt would be a feasible treatment if we consider the minor cerebrospinal fluid pathway which is the dominant cerebrospinal fluid dynamic at this age.