Gestational diabetes mellitus (GDM) is defined as glucose intolerance of various degrees with onset or first recognition during pregnancy. Women with GDM are at high risk of developing type 2 diabetes later in life, but the risk of developing type 1 diabetes is also increased. Positivity for glutamic acid decarboxylase (GAD) antibodies during pregnancy confers a high risk for subsequent progression to type 1 diabetes. Here, we reported the two cases with GDM who were GAD antibody positive and progressed to type 1 diabetes with different time-courses. One woman with GDM progressed rapidly to classical type 1 diabetes while the other became slowly progressive IDDM (SPIDDM) [or latent autoimmune diabetes in adults (LADA)].