BACKGROUND: Protracted Febrile Myalgia is a rare form of vasculitis that is diagnosed in patients with Familial Mediterranean Fever. OBJECTIVE: To present a case with Familial Mediterranean and Anklosing Spondylitis on anti-TNF therapy for three years, who developed protracted febrile myalgia syndrome. METHODS: Case report. RESULTS: A 35-year-old woman with known Familial Mediterranean Fever and Anklosing Spondylitis for 3 years presented with fever, diarrhea, intermittent abdominal pain and severe diffuse muscular pain lasting for two weeks. The patient was investigated for any infection focus. The patient was diagnosed as having Protracted Febrile Myalgia four weeks after the onset of the symptoms. Prednisolone 1 mg/kg per day was applied. Her fever and muscle pain resolved within 48 hours. CONCLUSION: The coexisting Ankylosing Spondylitis disease and the use of anti-TNF treatment in patients with Familial Mediterranean Fever could be a confounding factor for the investigation of fever. Steroid therapy has a dramatic response.