The opinion that parkinsonism is caused by damage of the substantia nigra was advanced by Brissaud 1 on the basis of the findings in a case of Charcot's studied by Bechet 2 and Blocq and Marinesco. 3 In a series of 54 autopsies Tretiakoff 4 found the substantia nigra degenerated in 16 categories of neurologic disorder including three cases of "catatonic" form of parkinsonism, nine cases of "typical" parkinsonism, and one case of unilateral parkinsonism. He concluded that damage of the substantia nigra is invariably accompanied by rigidity and tremor (and, if unilateral, that the effect was crossed) except when hemiplegia or poliomyelitis coexisted or when the patient was in extremis. Reviews of the literature 5-21 conducted since that time have substantiated a high positive correlation between nigral lesions and parkinsonism, but Tretiakoff's unitary theory of causation of parkinsonism has not been universally accepted because ( 1 ) neuropathologic changes have been