Macrophage activation syndrome (MAS) has been rarely reported in the course of adult-onset Still's disease (AOSD) and in the majority of cases, it was triggered by an infection. Here, we report, to our knowledge, the first case of MAS occurring after adalimumab treatment initiation and not triggered by an infection. A 26-yearold woman with classical features of AOSD developed persistent fever, severe bicytopenia associated with extreme hyperferritinemia, hyponatremia and abnormal liver function tow months after the initiation of adalimumab treatment. The diagnosis of MAS was made without histological proof. The patient was treated with methylprednisolone pulse therapy and her condition improved. During the disease course, extensive studies could not identify any viral infection or other known underlying etiology for the reactive MAS. The adalimumab was incriminated in this complication. Currently, the patient is in remission on tocilizumab and low-dose prednisolone.