It is often a challenge and a dilemma for clinicians encountering patients with pyrexia of unknown origin. Numerous tests performed to determine the underlying cause often give inconclusive results. We present a 52-year-old man with undulating fever for more than 10 months with persistent hyperferritinaemia, and negative immunological and serological markers. Despite corticosteroids, disease modifying anti-rheumatic agents and immune-modulator therapy, he succumbed to the illness. A diagnosis of refractory Adult onset Still's disease complicated by haemophagocytic lymphohistiocytosis was made.