A 17-year-old girl presented to her general practitioner with a 1-week history of fever, arthralgia, general malaise and dry cough. She had a history of systemic onset juvenile idiopathic arthritis (SOJIA), diagnosed at age 2 years and treated with aspirin, and had been in remission for 13 years. She was initially treated by her GP with oral roxithromycin, and admitted to hospital 3 days later with worsening of her symptoms. Her admission chest x-ray (Box 1) revealed bilateral perihilar infiltrates. A diagnosis of severe community-acquired pneumonia was made and broad spectrum antibiotics were commenced, including vancomycin, moxifloxacin, and oseltamivir. Despite this treatment, her condition deteriorated. On Day 3 of admission she required endotracheal intubation and circulatory support with noradrenaline 18–50 μg/kg/min, and was admitted to the intensive care unit (ICU). She remained hypotensive, with a mean arterial pressure of 50mmHg, and with sinus tachycardia of 132 beats/min, and subsequently required renal replacement therapy. She remained febrile for the first 3 days after admission (temperature range 37.5°C–39°C), and her temperature settled to normal after appropriate therapy was initiated. Investigations included a computed tomography scan of her abdomen, which showed hepatosplenomegaly, and liver function tests, which showed elevated conjugated bilirubin (36 mmol/L; reference range [RR], < 4mmol/L), γ -glutamyl transferase (84U/L; RR,< 24 U/L), lactate dehydrogenase (3540U/L; RR, 150–280U/L), alanine aminotransferase (108U/L; RR, 10–30U/L) and aspartate transaminase (388U/L; RR, < 30U/L). Other abnormal parameters were her haemoglobin level (93g/L; RR, 120–160 g/L), platelet count (65 x 109/L; RR, 150–400 109/L), white cell count (2.9 x 109/L; RR, 4.5–13 x 109/L), international normalised ratio (2.1; RR, 0.9–1.2) and fibrinogen level (0.9 g/L; RR, > 2.5 g/L). Elevated inflammatory markers included C-reactive protein (301mg/L; RR, < 5mg/L) and serum ferritin (50 500μg/L; RR, 7–140 μg/L). A transthoracic echocardiogram showed a left ventricular ejection fraction of 60%, a mild reduction in right ventricular contractility, and a right ventricular systolic pressure of 48mmHg. A full screen for sepsis was performed, including a nasopharyngeal aspirate, bronchoalveolar lavage, blood cultures and serological testing; all were unremarkable. Other immunological tests performed were for Mycoplasma pneumoniae antibodies, Streptococcus pneumoniae urinary antigen, Legionella pneumophila urinary antigen, herpes simplex virus, cytomegalovirus, Epstein–Barr virus (EBV) IgM and IgG, influenza A and B, H1N1 influenza RNA, respiratory syncytial virus, parainfluenza and adenovirus DNA, Q fever (Coxiella burnetti) IgM and IgG, and serological tests for hepatitis, dengue fever IgM and Leptospira IgM, all of which were non-reactive. Urinalysis revealed a white blood cell count of 140 x 106/L (RR,< 10x 106) and an erythrocyte count of > 500x 106/L (RR,< 10x 106/L) with no microbial growth on culture. A bone marrow aspirate with trephine was performed on Day 4 (Day 2 in the ICU), and EBV DNA was detected in the resulting sample using a qualitative DNA test. The patient’s failure to improve, in combination with her past history of SOJIA, hepatosplenomegaly and a very high ferritin level led to a preliminary diagnosis of macrophage activation syndrome (MAS). Immunosuppressive treatment in the form of high-dose methylprednisolone (10mg/kg daily) and intravenous immunoglobulin (1 g/kg daily) were commenced. Inotrope and ventilatory requirements improved within 24 hours of this treatment. On Day 9 of admission she was extubated, and was discharged home on high-dose steroids 8 days later with no complications. Her bone marrow aspirate histological examination showed haemophagocytosis which confirmed the diagnosis of MAS (Box 2).