A 15-year-old male developed features of Still's disease. He was treated, with incomplete response, with aspirin and corticosteroids. Some 10 months after the onset he contracted chickenpox and became gravely ill. A diagnosis of histiocytic medullary reticulosis was made because erythrophagocytic histiocytes were detected in marrow aspirate films. Necropsy studies showed systemic infiltration by benign-appearing histiocytes and led to the final diagnosis of the virus-associated haemophagocytic syndrome due to varicella-zoster infection in an immunocompromised patient.