No one can study carefully the remarkable book of Hoffman on “The Mortality Statistics from Cancer Throughout the World,” and Williams’ “Natural History of Cancer,” and Wolff’s “Die Lehre von der Krebskrankheit,” and the special volume concerning “Mortality from Cancer and Other Malignant Tumors in the Registration Area of the United States,” recently issued by the Bureau of the Census, without feeling that something more should be attempted to arrest the progress of this direful disease. This seems all the more necessary and proper in view of the gratifying decrease of mortality which has been obtained in tuberculosis, of 27.8 per cent from 1900 to 1915, by diligent and intelligent medical supervision. The problem of cancer is indeed a great one, but surely it is not to be solved by greater activity along the lines under which its mortality has steadily risen 28.7 per cent during the same period just mentioned, in which tuberculosis has fallen so greatly. If this death rate of both diseases should continue the same for fifteen years more, cancer would outstrip tuberculosis in its actual fatality. Reason would seem to indicate the necessity of a radical change in our point of view and a complete change in our line of treatment. In the text of these and former lectures I have endeavored to show why and how cancer should be regarded from its medical aspects, and to illustrate by a few cases some of the results which could be obtained from this line of procedure. There is absolutely no claim or suggestion that the cancer problem has been solved, but only an aim to put the _real cancer problem_ in such a light that others might follow and develop the subject in a manner fitting to the very great importance of the end so strongly desired by all, namely, the checking of the steadily rising morbidity and mortality of cancer. Laboratory studies are of practical value as they supplement and enlighten clinical observation. The microscope and test tube have accomplished much for medicine and with animal experimentation have undoubtedly rendered inestimable service in its scientific advancement. But divorced from the practical study of patients they may fail in the ultimate end desired. In these and the former lectures I have endeavored to indicate certain lines of scientific investigation along which much more laboratory effort is desirable, in order to determine more definitely the metabolic and blood conditions which lead up to cancer. These I have attempted to follow to a limited degree in many cases, and found them of great service in their management.