Cerebrospinal-fluid-filled interhemispheric cysts associated with callosal agenesis are relatively rare lesions, and have been subject to a varied and confusing terminology. From a pragmatic surgical point of view, we believe that the dorsal III ventricular cyst  and the giant interhemispheric cyst , although of different embryological origin, can be classified as one group. The most important condition that must be distinguished from interhemispheric cysts is the alobar form of holoprosencephaly. We describe the clinical symptomatology in 16 children who were surgically treated with a cysto-peritoneal shunt. The outcome, both neurologically and developmentally, was good in the large majority of cases, and compared favorably to similar cases in the older literature. It therefore seems reasonable, at the present state of knowledge and until further studies clarify the origin and natural history of these cysts, to treat them as early as possible in order to prevent gross developmental deficits.