Diabetes mellitus is a common medical disorder that will be encountered by every practicing dentist. Knowledge by the dentist of the general and oral signs and symptoms of undiagnosed or poorly controlled diabetes mellitus are essential, and patients displaying these signs or symptoms should receive medical referral. Patients suspected, or known to suffer from undiagnosed or uncontrolled diabetes mellitus should receive only emergency care until their health status has been properly evaluated. In the event the degree of control of a known diabetic is unknown or the patient is poorly controlled, antibiotic therapy should be administered in conjunction with any necessary surgical procedure or in the presence of oral infection. The practitioner must be prepared to manage diabetic emergencies should they occur in the dental office, and hypoglycemic incidents are most likely. The developing therapies now available for medical management of diabetes mellitus suggest that this condition will be more effectively controlled in the future, but medical consultation is important to proper periodontal patient management. New evidence suggests that advanced periodontal disease may interfere with diabetes mellitus control and the physician should be made aware of the patient's periodontal status. Under most circumstances, the well-controlled diabetes mellitus patient can receive safe and effective periodontal therapy with some modification of office protocol, and there is little reason to anticipate that the controlled diabetes mellitus patient cannot look forward to a lifetime of periodontal health if proper and timely periodontal therapy is rendered and the patient maintains effective oral hygiene measures accompanied by appropriate supportive periodontal recall therapy.