Juvenile Dermatomyositis (JDM) is a rare autoimmune disease in childhood. Distinction between muscle inflammation and a residual state can be difficult especially under immunosuppressive therapy and in active disease without correlating muscle enzyme tests or functional muscle scores. Our goal is to demonstrate the benefit of whole-body magnetic resonance imaging (MRI) as a diagnostic modality in the detection and management of JDM. One patient with JDM was monitored using clinical examination, muscle enzyme tests, muscle scores and whole-body MRI. During immunosuppression, the patient presented several times to our department without clear correlation between clinical picture, muscle enzyme tests and muscle scores. Whole-body MRI proved reliable in assessing the true state of the disease, thus providing invaluable information in the management of the inflammatory myopathy. This is of utmost importance for the therapeutic optimization in order to prevent further damage especially in children with active but subclinical disease.