After reviewing the literature on psychic disturbance in normal pressure hydrocephalus, the author reports on 26 cases of CSF drainage. Impairment of memory and concentration were always present in normal pressure hydrocephalus; behaviour disorders were very frequent and characterized by both types of frontal behaviour; the mental picture also comprised difficulties of calculation, writing, visuoconstructive praxis and, less frequently, impaired speech and reading ability. CSF drainage brought clinical improvement in 73% of the cases. Disorders of consciousness, mental function and behaviour regressed generally progressively as soon as the operation had been performed. Two major factors affecting therapeutic efficacity appear to be the patient's age and the length of time between onset of the symptoms and start of therapy.