Introduction: Familial Mediterranean fever (FMF) is a genetic disease characterized by recurrent febrile episodes and mostly by the inflammation of serous membranes.We presented our case whom presented with acute myositis and was diagnosed as FMF.Results: A six year and 5 mounth old girl complained of severe pain in her right leg and a gait while walking for a period of one week.It was learned from the history that the right ankle swelled and hiperemic macular rash were determined on lateral malleol area three years ago and the acute phase reactants were elevated at that time.There was no consanguinity and family history of autoinflammatory diseases.Physical examination of the patient revealed swelling, redness, thenderness, heat increase of on the calf muscles and limitation of extension of the knee joint.White blood cells, C-reactive protein (CRP), and eritrocyte sedimantation rate (ESR) were eleveated, muscle enzymes were normal levels.In the USG, the right popliteal fossa fat tissue was prominent and inflamed, and the neural structures were thick and edematous.Magnetic Resonance imaging of the thighs and right leg posterolateral muscles was consistent with myositis.After NSAID treatment, the clinic improved.However, the high CRP and ESR persisted for six mounths.There was no mutation on TRAPS gene.When the M694V homozygous mutation was detected in the FMF genetics of the patient, Familial Mediterranean Fever was diagnosed and colchicine treatment was started.Other autoinflammatory diseases genes could not studied.Acute phase reactans of the patient regressed after the treatment.The patient experienced arthritis attack at once for two years follow up. Conclusion:The patients who carried M694V homozygous mutations may presented nonclassical findings of FMF.