A 59-year-old Japanese woman developed diabetes mellitus without ketoacidosis in the presence of glutamic acid decarboxylase autoantibody (GADA) (24.7 U/mL). After the amelioration of her hyperglycemia, the patient had a relatively preserved serum C-peptide level. Her endogenous insulin secretion capacity remained almost unchanged during 5 years of insulin therapy. The patient's GADA titers normalized within 15 months. The islet-related autoantibodies, including GADA, are believed to be produced following the autoimmune destruction of pancreatic beta cells and are predictive markers of type 1 diabetes mellitus. Therefore, the transient appearance of GADA in our patient may have reflected pancreatic autoimmune processes that terminated without progression to insulin deficiency.