Bendii Syndrome was a degenerative neurological illness affecting a minority of elderly Vulcans. Initially it was characterized by wasting, weakness, fatigue, fever, and a gradual but accelerating loss of emotional control, with victims exhibiting sudden bursts of emotion. Diagnosis was made by culturing tissue samples taken from the patient's metathalamus. There was no known treatment, but the disease was the focus of ongoing medical research.

A dangerous side effect of Bendii Syndrome was that the loss of emotional control could be telepathically projected to others. Vulcans were able to resist the emotional projections, but if the afflicted person was in the presence of non-Vulcans, the emotions could cause outbreaks of violence. An attending Vulcan telepath could keep these projections under control, but when the sufferer was particularly stressed, the side effect could overwhelm such efforts.

Ambassador Sarek was afflicted with Bendii Syndrome at the age of 201, and his struggle with the condition ended two years later in 2368, when he died. (TNG: "Sarek", "Unification I")

While the name of the affliction was generally pronounced "ben-dye", Perrin (Sarek's second wife) pronounced it "ban-dee" on at least one occasion.
The illness can be viewed as analogous to Alzheimer's disease.
In the novel Avenger, Sarek's death was attributed to a poison which had symptoms mimicking those of Bendii Syndrome. During the novel, Spock is discovered to have been infected with the same poison, but Doctor Crusher is able to find a cure for the poison in time to save Spock before the symptoms become irreversible.