In Familial Mediterranean Fever (FMF), arthritis is the initial symptom in 25% of patients. Although FMF arthritis is acute and self-limited, in 5% of cases protracted arthritis usually affecting large joints such as knee may occur. In this report, two cases are presented who were initially diagnosed as septic arthritis, first of which had four and the second had two synovectomy operations with the diagnosis of septic arthritis. Later on they were diagnosed as FMF with detailed history. We aimed to emphasize the importance of diagnosis of FMF, which is based mainly on history and clinical features in order to prevent unnecessary operations and suffering of the patient.