Medically compromised children and adults are increasingly likely to seek orthodontic care as improved medical management creates more long-term survivors. For the majority, treatment of orthodontic problems is feasible, but special precautions usually are required. These include medical consultation to establish the patient's prognosis, maintaining a current knowledge of drug therapy, and modifications in office procedures. Patients with a history of multiple transfusions should be presumed to be hepatitis carriers until proved not to be, and special precautions to protect office staff members and other patients should be taken. Decreased resistance to infection is a common complicating factor in medically compromised patients. Dentists must therefore avoid mucosal irritation and carefully monitor periodontal health. The practitioner should be alert to side effects of drug treatment such as xerostomia and depressed immune response, and be aware of the particular features of the underlying disease. Bleeding disorders, which can be managed by replacement of missing clotting factors, do not contraindicate orthognathic surgery. The major contraindication is poor anesthetic risk, which almost always is true for patients having sickle cell anemia because of poor blood oxygen saturation. Because orthodontic treatment can provide positive benefits, it should not be withheld solely because of the presence of a serious medical problem. With appropriate management, successful orthodontic treatment can be done for most patients.