Imaging studies of 25 cases of agenesis of the corpus callosum with interhemispheric cyst were retrospectively reviewed at the University of California, San Francisco, and Harvard Medical School, Boston. A classification based on morphology was developed from the study of CTs of 6 patients and MRIs of 19 patients. Two major types are recognized, each with subtypes: Type 1 cysts formed from extension or diverticulation of the third or lateral ventricles; and Type 2 loculated cysts not communicating with ventricles.
Type 1 cyst subtypes are characterized as follows: la) associated with communicating hydrocephalus; lb) obstructive hydrocephalus caused by diencephalic malformation and blockage of fluid from 3rd ventricle to aqueduct; 1c) associated with hemisphere hypoplasia and microcephaly.
Type 2 cyst subtypes: 2a) multiloculated cysts associated only with callosal agenesis; 2b) associated with deficiency of falx, heterotopia, polymicrogyria, and Aicardi syndrome; 2c) associated with subcortical heterotopia; 2d) interhemispheric arachnoid cysts. The majority of patients were male, except those with 2b cysts. 
COMMENT. Callosal agenesis and interhemispheric cyst is a heterogeneous group of malformations, mainly in males, consisting of two major types, one with cysts that are formed by extensions of the ventricles, and another consisting of loculated cysts not communicating with ventricles. Many are complicated by hydrocephalus and macrocephaly. The MRI permits the identification of previously unrecognized malformations associated with agenesis of the corpus callosum.