The question whether cancer or any other disease is becoming more prevalent is one upon which statistics can throw much light. Indeed, it must be answered by the help of statistics, of it is to be answered at all. At the start distinctions must be recognized between sickness and death, diseases and causes of death, morbidity and mortality. The prevalence of a disease is measured by the number of times the disease occurs in a population group in a unit of time, or the morbidity. Thus, the prevalence of typhoid fever as a disease is stated as the number of cases annually per 100,000 persons. The prevalence of a cause of death is measured by the number of times a death from that cause occurs in a population group in a unit of time, or the mortality. Thus, the prevalence of typhoid fever as a cause of death is stated as the number of deaths from typhoid fever annually per 100,000 persons. The prevalence of either a disease or a cause of death through at least two periods of time must be measured before the increase or decrease of either can be determined. The deadliness of a disease, that is, the number of deaths in each hundred or thousand cases, sometimes called the case mortality or case fatality, commonly differs in different regions and changes from one period to another. In consequence the morbidity from a given disease is not a satisfactory index of its mortality, and changes in the morbidity between two periods seldom, if ever, run exactly parallel with those in mortality. Cancer is both a disease and a cause of death. The question whether the disease is increasing can be answered only by a well developed and trustworthy system of morbidity statistics. No country or city in the world now has such a system, because none has a good registration of sickness in the entire population. Consequently, no significant statistical study of the increase of cancer as a disease, or of cancer morbidity, has yet been written or can be. The present paper is concerned only with the alleged increase of cancer as a cause of death. But before approaching that topic it should be conceded that, strong as the argument is for the increase of cancer mortality, the argument for the increase of cancer morbidity is probably stronger. In other words, the case fatality of cancer is probably less than formerly. This would follow from the increasing proportion of operations to cases, in some of which the disease never recurs, in others of which the progress of the disease is stayed, affording in the longer interim an increased chance for the action of new causes of death. The same conclusion is supported by other improvements in methods of treating cancer, all tending to prolong the patient9s life and increase the chance of an intervening death from some other cause.