Introduction: Adult Onset Still´s Disease (AOSD) is a rare systemic inflammatory disease of unclear etiology, with low incidence and prevalence among the general population. AOSD is a common cause of fever of unknown origin (FUO) in up to 20% of cases. Due to the scarce knowledge about this disease and its diagnosis, it is usually unrecognized in the differential diagnoses, worsening the prognosis and increasing complications in some patients.Case presentation: This is the case of a 32-year-old female patient with prolonged febrile illness, who did not respond to the antimicrobial treatments previously established. She was diagnosed with AOSD according to the Yamaguchi criteria after an extensive exclusion process. She was treated with first-line treatment with corticosteroids, achieving satisfactory results.Conclusions: The diagnosis of AOSD is an exhaustive process. Regardless of the availability of cutting-edge diagnostic tools, the medical history of the patient and an adequate physical examination are the most important aspects to consider.