An 81-year-old woman with adenocarcinoma of the rectosigmoid presented with progressive muscle weakness and difficulty swallowing, with symptoms worsening following successful resection of the tumor. On examination, she had weakness primarily of lower limb proximal muscles, with no other abnormal findings. Laboratory tests showed significant elevation of creatine kinase, and EMG findings indicated myositis of the proximal muscles. While MRI showed increased signal intensity on T2-weighted images of the leg muscles, indicating inflammation, muscle biopsy found widespread necrosis with only weak and focal lymphocytic infiltration. A diagnosis of paraneoplastic necrotizing myopathy (PNM) was made, and immunomodulatory treatment initiated, with little response. Four months later she returned with exacerbation of symptoms, this time with skin changes pathognomonic of dermatomyositis (DM). A repeat MRI showed progression of inflammation, as well as necrotic foci. It is important to distinguish between paraneoplastic myopathies such as DM and PNM, with implications regarding treatment and prognosis.