BACKGROUND Anaplastic large cell lymphoma (ALCL) is a rare type of non-Hodgkin lymphoma (NHL). The most common extranodal sites of ALCL are skin, subcutaneous tissue, bone, lung, and gastrointestinal organs. This study reports a case of ALCL with multiple extranodal involvement, especially the whole body skeletal muscles, with the aim to share the imaging features of the ALCL including magnetic resonance imaging (MRI) and 18F-fluorodeoxyglucose positron emission tomography-computed tomography (18F-FDG PET-CT). METHODS A 54-year-old female patient presented with two-month history of bilateral shoulder pain, which had exacerbated for 6 days prior to admission. MRI scans revealed multiple hyperintense on T2-weighted image (T2WI) and marked inhomogenous diffuse or rim enhancement masses in shoulder muscles. The probable diagnoses must include metastatic carcinoma in the skeletal muscle, purulent abscess, soft-tissue sarcoma and lymphoma. For our patient, however, she did not have a history of cancer or hyperleukocytosis. 18F-FDG PET-CT was made for further evaluation and identified whether there is another related lesion. PET-CT image showed widespread FDG uptake lesions, including cervical/retroperitoneal lymphnodes, subcutaneous tissue, hepar and multiple groups of whole body muscles. METHODS An ultrasound-guided tissue biopsy was performed on the left cervical lymphnodes. Histological and immunohistochemical examination showed ALK- ALCL. METHODS Clinicians planned to give our patient systemic chemotherapy. RESULTS Our patient died of multiple organ failure four weeks after her first visit to our hospital. CONCLUSIONS This disease should be considered when patient presented diffuse muscle swelling in particular when a history of cancer and hyperleukocytosis was not supported. The presence of soft tissue masses in skeletal muscles on MRI scans, as well as multiple marked focal tracer uptake on PET-CT and immunohistochemical analysis of the mass, may help the recognition of ALCL and the state of illness evaluation, allowing for the appropriate treatment strategy to be initiated.