[Incidence of fever of difficult diagnosis observed from January 1990 to August 1998].

Design: We evalueted incidence, period and etiology of the fever of difficult diagnosis (FDD) among 169 cases of fever of unknown origin (FUO) observed from 1-90 to 8-98. Materials and methods: we used the general and orientative diagnostic criteria useful for identifying some pathologies and also for distinguishing between infectious and non infectious fevers. Only as last attempt to solve the problem did we adopt ex-adiuvantibus treatments. Results: FDD incidence was 29.58%; we identified four main categories after detailed diagnostic work up:1) Fictitious fevers and abitual hyperthermia; 2) Systemic vasculitis with atypical clinical features; 3) Rheumatic polymyalgia; 4) Steroid-responder fevers. Conclusion: FDD is not a rare event, but of complicated resolution, some remaining unsolved. Our experience suggests that we consider only a few elements at first to distinguish organic from sine materia fevers: age, history of recent surgery, general clinical picture and aspecific indexes of inflammation.