Epidemiological data suggest a strong association between aging, dementia and comorbidity such as cancer, chronic renal failure or undernourishment. These chronic conditions may lead to invasive diagnosis procedures as well as to difficult therapeutic management. When they occur in elderly patients with cognitive disorders or dementia, physicians and caregivers should apply specific care program. For example, if an adjuvant chimiotherapy is discussed for an old demented patient with cancer, informed consent and details about the treatment program should be carefully provide. At the onset of a chronic disease, the assessment of its prognosis as well as its impact on the autonomy or quality of life is particular when the patient is also demented. We discuss the specific characteristics about management of demented elderly patients who require high risk treatment because of severe and lethal diseases.