The clinical and laboratory features of 47 cases of macrophage activation syndrome (MAS) were reviewed in a workshop within the Groupe Français d'Hématologie cellulaire. There was no predilection for a particular age group, while common symptoms at presentation included fever, hepatic and splenic enlargement and profound depression of blood count. Examination of bone marrow aspirates allowed diagnosis to be established in almost all cases. The most characteristic sign of MAS was the presence of well differentiated macrophages without notable cytologic abnormalities but shown to be actively ingesting haematopoietic elements. Haemophagocytic syndromes generally occur in patients who develop infections in the context of preexisting immunologic abnormalities or neoplasms. In the majority of patients evolution of the disease was regressive, once spontaneously but often after antibiotic, antiparasitic and/or antiviral treatment accompanied or not by corticotherapy and/or chemotherapy. Some regressive phases were followed by more or less long term relapse, especially in the case of associated systemic lupus erythematosus. There exists at present no explanation for the occurrence of MAS, although one may remark its association with other pathologies, in particular congenital or acquired immune deficiencies and haemopathies. Several hypotheses have been proposed to explain the appearance and evolution of the disease and at present two pathways of investigation of MAS seen to merit attention: exploration of macrophages themselves and their secretion products and exploration of lymphocytes and NK cells. The current possibilities for these investigations should lead to a greater understanding of the physiopathology of MAS and it is to be hoped that a better application of appropriate therapy will enable control of its evolution.