Congenital arachnoid cysts are commonly located at sylvian cistern or middle cranial fossa which are usually asymptomatic and incidentally found. Posterior fossa cysts, however, are usually large when diagnosed, and symptomatic. Three cases of large posterior fossa cysts were recognized on the diagnostic MRI investigation for infantile spasm, developmental delay, and the precocious puberty. Surgical decompression of the cysts by craniectomy, cyst excision and fenestration were performed successfully in two patients with arachnoid cysts in the cerebellopontine cistern and the suprasellar, right cerebellopontine, and prepontine cisterns, but an additional cystoperitoneal shunt was needed in a case with the cyst in the quadrigemial cistern with obstructive hydrocephalus. Infantile spasm was treated with vigabatrin and pyridoxine, and the true precocious puberty was managed with LHRH analogue(Decapeptyl ).