BACKGROUND: The cavum septum pellucidum is a potential space between the two leaflets of septum pellucidum. It is a normal anatomical variant seen in premature and newborn infants and usually disappears with brain maturation. Although moderate cystic dilatation of the cavum septum pellucidum may sometimes be observed, a true cyst is extremely rare with only a handful of reported cases, mostly in children and adolescents. CASE REPORT: We present a case of arachnoid cyst of the cavum septum pellucidum diagnosed on MRI in a 10 year old girl with the complaint of headache and decreased scholastic performance. CONCLUSION: Accurate diagnosis of the arachnoid cysts of the cavum septum pellucidum is important as they may cause serious symptoms which can be reversed by surgical treatment