Two cases of complete agenesis of the corpus callosum each with large interhemispheric cysts are presented. The first case is an adult patient with chronic renal failure secondary to adult polycystic renal disease who was neurologically asymptomatic until she had a seizure during hemodialysis. The second case is an infant, who was diagnosed in utero as hydrocephalic, with severe mental and motor retardation and intractable seizures. The clinicopathologic findings in the two cases are presented, along with a discussion of the possible etiologies and clinical significance.