Progressive supranuclear palsy (PSP) was originally described in 1964. Although some contended it was merely a variant of Parkinson's disease, a specific electron microscopic finding of straight, rather than twisted, filaments in the neurofibrillary tangles established PSP as a distinct entity. The almost pathognomonic early clinical finding of paralysis of downward gaze is due to lesions involving the lateral portions of the rostral interstitial nucleus of the medial longitudinal fasciculus. Recent neurochemical studies have identified both a decrease in central dopamine and acetylcholine. The etiology of PSP is unknown, and the therapy is generally ineffective.