Arachnoid cysts (AC) located within the interhemispheric fissure are rare, frequently associated with agenesis or hypogenesis of the corpus callosum. The optimal method for surgical treatment is under debate. In this study, we retrospectively reviewed the clinical results of seven infants (five males, two females; mean age, 5.1 months) with large interhemispheric AC who underwent cystoperitoneal shunting (CP) and evaluated the long-term efficacy in terms of clinical data, psychomotor development and neuroimaging. All patients were symptomatic and operated on before they reached 1 year of age. In three patients, additional ventricular catheters were inserted due to concomitant hydrocephalus. The additional catheters were joined with the cyst catheters and the peritoneal catheters with Y-shaped connectors in the same session. The patients were followed up for an average of 57 months (range 24-120 months). The clinical and developmental outcome was good in most patients with complete resolution of symptoms and signs. The developmental outcome scale, which included cognitive and psychomotor development, was "good" (normal) or "fair" (near normal) for six of seven patients. Radiologically, the cyst was completely or nearly totally resolved in five patients, and partially resolved in two. In these symptomatic patients with interhemispheric AC, CP shunting provided favorable treatment outcomes with complete or near-total resolution of symptoms and reduction in cyst size or disappearance of the cyst. (C) 2009 Elsevier Ltd. All rights reserved.