In my early work at the Philippine General Hospital, Manila, I was impressed with the frequency of cheek cancer in women, and also with the almost universal habit in elderly persons of chewing buyo. The possibility of buyo as an etiologic factor in mouth cancer occurred to me, and accordingly I have studied all the cases at my disposal and reviewed the histories of all cases of mouth cancer that are recorded at the Philippine General Hospital. The histories which I reviewed are of patients who were under the care of Drs. John R. McDill, P. K. Gilman, R. Rembe, Hans Ernst Schiffbauer and of patients from my own service. The photographs for the most part were made by the Bureau of Science. In presenting this paper, I wish first to discuss in some detail the various constituents of the buyo chew; secondly, to discuss the prevalence of the habit