RESUMEN
Aims: Our aim was to present a case of pyomyositis and highlight the need to distinguish it from a soft-tissue sarcoma which it can closely mimic. Presentation of Case: We report a 51-year old diabetic patient who presented with a painful swelling in the right upper arm, which was not associated with fever. The mass was well defined, tender and firm but lacked other signs of inflammation such as redness or increased local warmth. Even the magnetic resonance imaging (MRI) findings were more in favor of a soft-tissue sarcoma. A diagnosis of pyomyositis was revealed when the trucut biopsy performed, yielded frank pus discharge. Discussion and Conclusion: This case illustrates a diagnostic quandary of a soft tissue mass. There may be a paucity of inflammatory signs in patients with an underlying immunocompromised state, masking an initial diagnosis of pyomyositis. Although softtissue sarcomas generally present as painless lumps, they can be painful especially with larger tumors that are increasing in size, further adding to a diagnostic dilemma. While MRI is a valuable tool for early diagnosis of pyomyositis, it can still be misleading. Prompt biopsy is recommended to facilitate the correct diagnosis.