Dementing illnesses are chronic disorders causing global impairment of intellectual function. The most common cause is Alzheimer's disease, and this, together with most other forms of dementing illness, is relentlessly progressive. Frequently, however, there is an abrupt deterioration in physical, behavioral, or cognitive function in someone who is known to suffer from a dementing illness. This article outlines an approach to the many factors capable of producing such acute decompensation.