Recently, the relationship between digestive diseases and oral diseases has been a focus of interest. Physicians and dentists should not simply diagnose diseases without understanding the possible relationships between the mouth and digestive system, such as double cancers in oral cavity and esophagus or stomach. Physicians should learn to examine the mouth systematically. On the other hand, dentists should not only treat dental caries, periodontitis, and denture problems, but should also study medical knowledge in order to develop dental treatment for total oral health care. In other words, dentists should become oral physicians or stomatologists. In this paper, we describe how digestive diseases relate to oral diseases and the important systemic findings you can deduce from oral findings. We hope that the knowledge presented here will be helpful for dentists and physicians in their daily practice.