The mouth is a unique site, due to the presence of hard and soft tissues in close approximation. It is an important entry point for many pathogens in the body. Many systemic diseases manifest in the oral cavity and mouth can show early signs or the only signs of a disease process at a site elsewhere. Periodontitis has been associated with a number of systemic diseases such as diabetes, cardiovascular diseases, respiratory diseases, bone disorders, kidney disease and drug allergies. Periodontal disease might influence the morbidity and mortality of systemic diseases, constitutes a research topic of great current interest. A high percentage of dental patients have medical conditions that could affect the clinician's therapeutic decisions. This may be of an even greater concern for periodontal patients, who appear to have a higher prevalence of systemic diseases than patients without periodontal disease. Identification of all past and present medically compromising factors in periodontal patients is essential for their total well- being, as well as their oral health care. A dentist thus can frequently be exposed to such conditions and play a key role in the diagnostic procedure.