The date and site of onset of the great pandemic are subjects concerning which there is no conclusive information. There have been small outbreaks of clinical influenza with epidemic tendencies at one place or another during nearly all of the intervening years since 1889. In all of them the question is open as to whether they were true influenza, and also assuming that some were true influenza, how many of them should be so included. There are some who believe that the increase of morbidity following the measles epidemic in the United States Army camps in the winter of 1917–18 is genetically associated with the great pandemic. In short, there is no one point in the last few years at which we may say that influenza which had previously been non-existent started at a focus and spread throughout the world. It follows from the experience of 1889 that we should at least attempt to find an endemic focus and to follow the progression of the disease. It is safe to say that once having become pandemic the disease spread as it did thirty years previously. Experience in this country, where the autumn spread began in the New England States and continued West and South; knowledge of the late spread to remote localities; the fact that the disease first appeared in England, etc. in sea coast towns; the introduction of the influenza into new countries at seaport towns, after the arrival of infected ships, all coincide well with the past history. But which of the several local epidemics of the preceding years was the direct progenitor of the great pandemic? In order to follow more clearly the development of the facts we will record here the various hypotheses that will come up for consideration as to the site of origin of the disease. 1. Influenza is endemic in some one locality, such as Turkestan in Asia, from which place the disease spreads throughout the earth at intervals, after having acquired in some way greatly increased virulence. The local outbreaks of interepidemic times are not due to the virus which causes the great pandemics and should be called pseudo-influenza in contrast to influenza vera. Following the pandemic it is true, however, that for a succession of years local outbreaks occur, due to the pandemic virus which has been left deposited in small endemic foci. These disappear in the course of a few years. 2. The second hypothesis is similar to the first, except that in it is considered the possibility of there being more than one endemic focus, at least two, one in the old world and one in the new. Although Leichtenstern believed in the first hypothesis he did not deny the possibility of the second.