Inflammatory myopathies are a heterogeneous group of conditions characterized by proximal muscle weakness, nonsuppurative inflammation of skeletal muscle, with elevated muscle enzyme levels and characteristic electromyography and muscle biopsy findings. The authors describe a clinical case of a young woman, admitted with a four day history of bilateral thigh myalgia. She was afebrile and without skin, mucosal or joint involvement. Thigh muscle palpation was painful. Complete blood count revealed leukopenia and thrombocytopenia. High levels of creatine kinase, serum aminotransferases and myoglobin were detected. Metabolic, toxic and drug-related causes were excluded as well as infectious diseases, malignant tumours and endocrine myopathies. Auto-antibodies for connective diseases were negative. Magnetic resonance imaging and electromyography of lower limbs were suggestive of inflammatory myopathy. Generalized muscle weakness and dysphagia were reported subsequently. Clinical and laboratorial improvement was seen after corticotherapy. Muscle biopsy revealed myopathy signs without inflammatory changes or vasculitis. After prednisolone reduction, presently without treatment, she remains asymptomatic with normal laboratorial findings. The authors emphasize in this case of inflammatory myopathy the unusual clinical and laboratory evolution and the importance of a cautious differential diagnosis.