A rare case of arachnoid cyst in the trigone of a lateral ventricle is reported. The patient was an 8-year-old boy who had had four episodes of convulsions prior to admission. Computed tomography (CT) and magnetic resonance imaging (MRI) demonstrated a cystic lesion containing fluid resembling cerebrospinal fluid. Although he received a cyst-peritoneal shunt, the lesion did not decrease in size. Direct removal of the cyst was then scheduled. The entire cyst was finally removed, although it was firmly attached to the choroid plexus. The enlarged trigone gradually decreased in postoperative CT. The effectiveness of a cyst-peritoneal shunt is not always satisfactory. We recommend total resection of the cyst without use of a shunt system.