Pyomyositis is a disease characterized by primary bacterial infection of the skeletal muscles of lower limbs and thorax. It is a very rare disease (100 cases in USA in the last 20 years) but with an increasing occurrence especially among patients with AIDS in an advanced stage or in association with other pathologic conditions like diabetes mellitus, cancer, connective tissue diseases or hematological disorders, long usage of corticosteroid drugs. We are hereby showing the case of a 46-year-old female who presented difficult problems of differential diagnosis until final diagnosis was established. Her life was saved due to an exemplary interdisciplinary collaboration and by using an extremely useful imaging method for musculoskeletal pathology--IRM. The IRM protocol: coronal and axial T2 weighted SE, coronal and axial T1 weighted SE, axial and coronal T2 weighted + Fat Sat was comprehensive and conclusive. The imaging diagnosis established the presence and extension of the muscle abscess, leading to an efficient drainage. After the surgery, the repeated IRM confirmed the drainage efficiency. Unlike the French literature which claims that the muscle abscesses are secondary to a primary center, in this case, and in accordance with Anglo-Saxon literature, the muscle abscess was primary, in absence of any other center of infection.