In April 1917, Dr Constantin von Economo presented his clinical and pathologic findings of a new disease—soon to be part of a worldwide epidemic—before the Vienna Psychiatric Society. He named itencephalitis lethargica. After years of careful observation, he collected and analyzed thousands of cases and classified them into 3 clinical syndromes: somnolent-ophthalmoplegic, hyperkinetic, and amyostatic-akinetic forms. He described the now legendary postencephalitic Parkinsonism, noting that symptoms could emerge years after the original infection, often without signs of prodromal "flu." He emphasized the neuropathologic findings: inflammatory changes in the tegmentum of the midbrain accounting for the sleep disturbance and ocular signs. After encountering sporadic cases following the epidemic, he concluded that the somnolent-ophthalmoplegic syndrome was the primary expression of encephalitis lethargica. This article outlines the observations and conclusions of Dr von Economo during and after the epidemic through seminal quotations primarily from his published works, as well as from more recent reports.