A 13-year-old right-handed boy sustained minor head trauma in a traffic accident. Computed tomography of the head showed no injuries, but revealed an arachnoid cyst in the left middle fossa. Single photon emission computed tomography revealed relatively decreased cerebral blood flow (CBF) in the left frontal and temporal lobes compared with the contralateral regions. The Wechsler Intelligence Scale for Children-Revised (WISC-R) test showed full scale intelligence quotient (IQ) was normal, but also a discrepancy between verbal IQ and performance IQ. Cyst-peritoneal shunting was performed. Postoperatively, laterality of the CBF in the frontal and temporal lobes was resolved and the verbal IQ/performance IQ discrepancy was normalized. In this case, although the WISC-R score was apparently within normal limits, latent regional ischemia induced by compression due to the arachnoid cyst may have caused neuropsychological dysfunction. CBF study and neuropsychological evaluation by the WISC-R may be useful for selecting patients with apparently asymptomatic arachnoid cysts for surgery.