Fever of unknown origin, pleural and pericardial effusions can be caused by a variety of independent agents. On the other hand, we can identify a common causative condition in other cases. Infectious diseases, malignancies and autoimmune diseases are the most common etiological factors. Considering the pleural and pericardial effusion, we also have to think of cardiovascular and pulmonary diseases. The basis of every diagnostic process is thorough medical history and detailed clinical examination followed by laboratory and imaging methods. In spite of that, the right diagnosis sometimes stays long time hidden. One of such conditions is Adult-onset Stills disease (AOSD). It is a rare inflammatory, potentially life-threatening disease with unclear pathogenesis and heterogeneous symptoms. It has some features similar to systemic form of juvenile idiopathic arthritis. Diagnosis is established so called per exclusionem by fulfilling a set of clinical criteria and ruling out other diseases with similar symptomatology. In our article, we present an example of such an arduous diagnostic journey to final diagnosis.