Clinical and pathological experience has shown that encephalitis lethargica, like neuro-syphilis and disseminated sclerosis, is essen tially a chronic disease. For whether it declares itself acutely or insidiously, and however complete the recovery from the initial phase may appear to be, in a large proportion of cases the infection, which has evidently lain dormant for weeks, months or years, again becomes active and gives rise to the grave disabilities with which we are familiar. The distressing result is that we never know when the patient is cured. At present we are at a prognostic impasse.