Fever of unknown origin is defined by fever lasting more than 3 weeks, with no diagnose despite a consequent work-up. 4 subgroups were determined: infectious fever, such as subacute bacterial endocarditis, tuberculosis or intra-abdominal abcess, inflammatory diseases such as temporal arteritis or adult Still's disease, neoplastic disorders, and miscellaneous causes. About one third of patients have no diagnosis at the end of the work up, and for those the prognose is usually good. There is no algorithm to know which tests to order: focused history and detailled physical status give important clues to decide about additionnal laboratories testing. An empirical treatment is justified only in case of vital risk.