Unilateral hydrocephalus due to occlusion of one foramen of Monro was found incidentally at necropsy in a 52-year-old man. There was no evidence of a postinflammatory or neoplastic origin of the occlusion. Backward tilting of the diencephalon, asymmetric insertion of the septum pellucidum, lateral tilting of the fornices, and deformation of the hippocampal formation indicated an early developmental origin of the lesion.