We report a patient who presented with clinical and MRI findings suggestive of polymyositis but, in whom, muscle biopsy disclosed a strikingly different diagnosis. A 65-year-old woman presented with 3-week history of bilateral proximal muscle pain and weakness. Laboratory investigations showed markedly elevated inflammatory markers and mildly elevated muscle enzymes. MRI scans of lower limbs showed features suggestive of polymyositis. However, muscle biopsy showed features of a polyarteritis-type vasculitis affecting an intramuscular blood vessel. Our reports highlight the critical role of muscle biopsy in establishing the correct diagnosis in patients with suspected myositis.