Other than Alzheimer's disease normal pressure hydrocephalus (NPH) is the commonest cause of dementia. It affects millions. The diagnosis is missed more than 80% of the time. Millions of patients with have been wrongly diagnosed. If they have idiopathic normal pressure hydrocephalus they can be treated and possibly cured. The author describes the symptoms of idiopathic normal pressure hydrocephalus and how it can be diagnosed and treated. He also explains why doctors and nurses so often miss this diagnosis and why idiopathic normal pressure hydrocephalus is so little known.