Bendii Syndrome was a degenerative neurological illness very rarely affecting Vulcans over the age of two hundred.

The early symptoms of the disease included sudden bursts of emotion, often irrational anger, and eventually all emotional control would be lost. Diagnosis was made by culturing tissue samples taken from the patient's metathalamus, though the results of this test took several days to arrive.

Ten Forward fight

The catastrophic results of emotional transference

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 anger and 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.

According to Perrin, Bendii Syndrome was considered more a folk tale than a disease by 2366, with no case having been diagnosed since at least 2165.

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 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 Beverly Crusher is able to find a cure for the poison in time to save Spock before the symptoms become irreversible.