In patients with idiopathic thrombocytopenic purpura (ITP), which is considered an autoimmune disease, eradication of Helicobacter pylori cures a significant number of patients. However, here we report an adult patient who developed type 1 diabetes (T1D), which is also considered an autoimmune disease, after eradication of Helicobacter pylori. Although further investigation is needed to understand the pathophysiology of T1D, we would like to emphasize that clinicians should consider the risk of its development after eradication of Helicobactor pylori.