In this study we aimed to investigate the findings in patients with adult-onset Still's disease (AOSD) admitted with fever of unknown origin (FUO) during the last 18 years in our unit, in order to discover the ratio of such patients to all patients with FUO during the same period, and to determine the clinical features of AOSD in FUO. The number and the aetiologies of the patients with FUO diagnosed between 1984 and 2001, and the clinical features of those with AOSD, were taken from the patient files. The diagnosis of AOSD was reanalysed according to the diagnostic criteria of Cush et al. . The presumed diagnoses before a diagnosis of AOSD was established were also noted. The chi(2) and Fisher's exact tests were used for statistical analysis. We studied 130 patients with a diagnosis of FUO, 36 (28%) of whom had collagen vascular diseases. Of these 36 patients, 20 (56%, 12 female, 8 male, mean age 34 years, range 16-65) had AOSD. Clinical and laboratory findings were as follows: fever (100%), arthralgia (90%), rash (85%), sore throat (75%), arthritis (65%), myalgia (60%), splenomegaly (40%), hepatomegaly (25%), lymphadenopathy (15%), anaemia (65%), neutrophilic leukocytosis (90%), increased erythrocyte sedimentation rate (100%), elevated transaminase levels (65%), a negative RF (100%), and a negative FANA (80%). Antibiotics had been prescribed in 18 (90%) of cases. The presumed infectious diagnoses were streptococcal tonsillitis/pharyngitis (50%), infective endocarditis (four patients), sepsis (two patients) and acute bacterial meningitis (two patients). The presumed non-infectious diagnoses were acute rheumatic fever (three patients), seronegative rheumatoid arthritis (two patients) and polymyositis (two patients). Sixteen patients were followed for a mean duration of 30 months (range 2-59). A remission was obtained with indomethacin in three cases (19%), and with prednisolone in the remainder. Relapse was detected in three cases (19%). AOSD is one of the most frequent aetiologies of FUO. During the diagnostic course of a patient with FUO, a maculopapular rash and/or arthralgia and/or sore throat should raise the suspicion of AOSD. Because the disease has heterogeneous clinical findings, certain bacterial infections (e.g. streptococcal pharyngitis and sepsis) are generally considered and the prescribing of antibiotics is common.