The Authors report a case of Reye's syndrome associated with an hemophagocytic histiocytosis in a 16-month-old baby. The vaccination against measles, mumps and rubella administrated 15 days before the onset of symptoms is suggested as the etiologic factor of both diseases. The Authors believe that the vaccine, composed of alive viruses was responsible of immunodepression and, as a common viral infection, of Reye's syndrome and hemophagocytic histiocytosis in the child constitutionally predisposed.