"Parkinsonian states" comprise a large group of morbid conditions possessing a fairly well defined clinical picture—characteristic rigid posture, with expressionless face, retardation of voluntary movements, hypertonia and tremor without signs of pyramidal tract lesion. Here belong Parkinson's disease (paralysis agitans) and so-called postencephalitic parkinsonian syndromes. The similarity between the two is so great that some consider them, clinically as well as pathologically, identical morbid entities. Netter1 maintains that in certain instances Parkinson's disease is caused by the virus of epidemic (lethargic) encephalitis which is responsible for its sequelae ("accidents tardifs"). Souques2 is more emphatic. He looks on both the classic and postencephalitic types as clinical manifestations of one disease—paralysis agitans, the difference being in the degree of the lesion. He asserts that when the encephalitic process in the mesencephalon is slight and "reparable," a parkinsonian syndrome is produced; if the lesion is irreparable, paralysis agitans results. The latter