An 18-year old Japanese woman with thirst, weight loss and palpitations for a few weeks’ duration was admitted. Her diagnosis was diabetic ketoacidosis with acute-onset type 1 diabetes (T1D) and Graves’ disease (GD) (autoimmune polyglandular syndrome type 3 variant; APS3v). She was treated with intensification therapy with insulin and anti-thyroid drugs. Human leukocyte antigen testing revealed that her HLA type was HLA- DRB1*08:02-DQA1*03:01-DQB1*03:02 haplotype, which was vulnerable not only to APS3v, but also to both acute-onset T1D and GD. This genetic factor probably contributes to the simultaneous occurrence of T1D and GD.