Polymyositis (PM) is an autoimmune disease which affects skeletal muscles. In young age, it usually occurs as an idiopathic disorder associated with specific autoantibodies (anti-Jo), while in older age it is often associated with neoplasms. It can present with symptoms of other autoimmune diseases, such as systemic sclerosis (SSc), a rare progressive disease characterized by collagen deposits in various tissues and organs. A 65-year-old patient, long-time smoker, came to the ER because of painful edema in the distal parts of his limbs and proximal muscle weakness of his arms and legs. Although his muscle enzymes were not increased, PM was confirmed by the characteristic pathohistological finding. The patient had sclerodermal skin lesions on his back, but he did not have other typical SSc symptoms, and the specific autoantibodies were negative. He received glucocorticoid therapy (GC) after we had finished screening for malignant tumors. He felt better, his muscle strength returned, and the limb edema disappeared. Four weeks later, he developed symptoms which are more typical of SSc, such as dysphagia, Raynaud’s phenomenon, and skin thickening of the limbs that had been swollen. PM is often associated with SSc. It is not clear if the exacerbation of latent SSc was stimulated by GC, or if it was just a simple overlap of the two diseases with different onsets. There are no therapy guidelines for the treatment of this combination of diseases. Careful use of GC is necessary even if SSc symptoms are discreet, because of the well-known effects of GC in SSc.