Abstract: We report a case of adult-onset Still's disease (AOSD) presenting with fever, myalgia, and pleural effusion. The patient exhibited significantly elevated inflammatory markers and exudative pleural effusion with markedly elevated leukocyte count showing neutrophilic predominance. After comprehensive exclusion of infectious etiologies, the diagnosis of AOSD was established. Clinical symptoms resolved following anti-inflammatory therapy with glucocorticoids and tocilizumab. This case highlights a critical clinical insight: while infection is a common cause of elevated inflammatory markers, it is not an invariable etiology. In such presentations, meticulous clinical observation, thorough diagnostic screening, and systematic inference are essential for accurate diagnosis.