Although clinical trials of etanercept in adult and juvenile rheumatoid arthritis have generally revealed few adverse events, significant concern has arisen over the potential evolution of secondary autoimmune disease due to modulation of tumor necrosis factor. There have been few reports of such diseases developing, and none in children receiving this therapy. Reported herein is the case of a 7-year-old girl with a 3-year history of systemic-onset juvenile rheumatoid arthritis with a polyarticular course, in whom type 1 diabetes mellitus developed 5 months after the initiation of etanercept therapy.