Elderly individuals often have different causes of fever of unknown origin compared to young persons. While infections dominate in the young, system diseases like temporal arteritis and polymyalgia rheumatica, malignancy and drug fever are common causes in the elderly. The symptoms may be blunted with lack of fever. Co-morbidities may complicate, dominate, and confuse the clinical impression. In the diagnostic work-up it is important to consider other co-morbidities and overall frailty. Patients should be informed and their wishes taken into account in the early phase of planning diagnostic work-up and treatment.