A 42-year-old woman suffered shortness of breath and whole-body pain, more prominent in both thighs. After the initial assessment, a paraneoplastic syndrome was considered clinically, and FDG PET/CT was performed to search culprit malignancy. The images did not reveal any focal lesion typical of solid hypermetabolic malignancy. Instead, diffuse increased muscle activity was noted in most musculatures, with some focally intense. The findings were suggestive of polymyositis, which was subsequently confirmed.