B AC K G R O U N D This study aimed to find out predictors of severe outcome and contributions of different methods to diagnosis in fever of unknown origin (F U O). M ET H O D S Consecutive 154 patients, 83 males and 71 females ( mean age 42±17), who met the modified Pedersdorf criteria for FUO, were enrolled. Death, malignancy, severe infection, and weight loss >10%, were considered as severe disease (SD) outcome and other outcomes referred to non-severe disease (NSD). R E S U LT S Fifty-three (34.4%) patients had infections, 47 (30.5%) had non-infectious inflammatory diseases (NIID), 22 (14.2%) had malignant diseases, and 8 (5.2%) had miscellaneous diseases. In 15.6% (24 cases) of the cases, the etiology remained obscure. Sixty three (40%) of the cases had SD (22 malignant, 19 infectious, 12 NIID, 7 undiagnosed, 3 miscellaneous diseases) and 91 (60%) had NSD. None of the clinical symptoms was related to outcome severity. Low hemoglobin levels, elevated leukocyte counts and increased C-reactive protein levels indicated SD with univariate analysis (p<0.05). However, none of the parameters was independent predictor of severe outcome. Rates of positivity and contribution to diagnosis of the methods were; chemical techniques (36%-1.6%), biopsy (30%-95%), i m m u n o l o g i ca l serology (13%-40%), microbiologica l serology (4%-50%), imaging techniques (30%-23%) and culture techniques (5%-42%), respectively. CO N C L U S I O N S In FUO presence of the anemia, leukocytosis and increased CRP levels may be signs of possible severe outcome.