The initial definition for FUO was proposed by Petersdorf and Beeson in 1961 : "fever > 38,3°C, permanent or recurrent, for more than 3 weeks, without diagnosis after one week hospitalisation and explorations". This definition has been updated by Durack and Street in 1991 who distinguish "classics FUO" from 3 other categories : nosocomial, patient with HIV and neutropenia. They also choose an other way of diagnosis : 3 days of investigations in hospital OR in consultation.