Persons with overwhelming damage to the cerebral hemispheres commonly pass into a chronic state of unconsciousness (ie, loss of self-awareness) called the vegetative state. When such cognitive loss lasts for more than a few weeks, the condition has been termed a persistent vegetative state, because the body retains the functions necessary to sustain vegetative functions. Recovery from the vegetative state does occur, but many persons in persistent vegetative states live for months or years if provided with nutritional and other supportive measures. The withdrawal of life support from these persons with loss of higher brain function is a controversial issue, as highlighted by public debates and judicial decisions. This article provides criteria for the diagnosis of permanent unconsciousness and reviews the available data that support the reliability of these criteria. Significant legal decisions have been made with regard to withdrawal of life support to patients in persistent vegetative states, and the trends in this area are discussed.