Adult onset Still's disease is a rare systemic inflammatory disorder. At the onset of the disease sore throat, pharyngitis; which does not respond to antibiotics, one or two times peaking febrile episodes, marked salmon-colored rash on the trunk and extremities, arthralgia, arthritis, myalgia, fatigue, loss of appetite with nausea and weight loss; hepatosplenomegaly and lymphadenopathy can be seen. Among laboratory examinations levels of ferritin and other acute phase reactants distinctly rise, and neutrophilic leukocytosis; ANA and RF negativity are detected. Pleural and pericardial effusions, transient pulmonary infiltration, and rarely myocarditis can be seen during the course of the disease. Here we report a patient who was examined for fever of unknown origin and diagnosed with adult onset Still's disease which is a rare etiology of pleural effusion.