Macrophage activation syndrome MAS describes the clinical, biological and histological symptoms related to a probably T lymphocytes/NK cell driven stimulation of macrophages with the consequence of a hemophagocytosis involving numerous organs, preferentially bone marrow, explaining the other term of "hemophagocytic syndrome". Clinical symptoms include cytopenia, multiple organ dysfunction, fever unresponsive to antibiotics, fatigue and rash. Infections (bacteria, virus or parasites), lymphoproliferative disorders, cancers, systemic diseases are the most prevalent triggers or etiologies of M.A.S. Evidence of haemaphagocytosis is obtained in the majority of cases with bone marrow specimens. In some cases haemophagocytosis can spare the bone marrow with involvement confined to other tissues such as liver and spleen. Very high levels of ferritine seem to correlate well with the presence of haemophagocytosis and is a possible marker for an early diagnosis. Early treatment initiation is mandatory. Corticosteroids, cytostatic drugs such as etoposide, cyclosporine A, plasmapherese, intravenous immunoglobulins and anti TNFalpha are proposed but no randomized trials were published.