Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization and treatment of sex hormone-dependent cancers due to their ability to bring about 'chemical castration'. Recently it has been showed that it also plays an important role in the modulation of the immune system. We report a case of vasculitis and histologically proven severe polymyositis developed 5 days after GnRH administration due to ovulation induction in an infertile patient with no previous history of autoimmune diseases. After treatment with corticosteroids (prednisone, 30 mg daily), aspirin, and pentoxyphyllin in combination with rehabilitation, an amelioration of muscle impairment was observed.