Ketosis-prone diabetes mellitus (KPD), which is an atypical type of diabetic mellitus with severe β cell dysfunction, is accompanied by ketosis or ketoacidosis without specific preceding factors at diagnosis. KPD shows mixed features of type 1 and type 2 diabetes. In some cases, the recovery of the function of β cells during intensified diabetic management enabled the termination of insulin therapy. The Aβ classification system classifies KPD patients into four distinct subgroups depending upon the presence or absence of β cell autoimmunity and β cell functional reserve and has been recognized as an important tool to predict clinical outcomes. In Korea, several cases of KPD with absence of β cell autoimmunity have been reported. A 60-year-old man presenting with DKA (diabetic ketoacidosis) as the first manifestation of diabetes, was shown to have β cell autoimmunity. A significant improvement in glycemic control was shown as a result of aggressive diabetic management; shortly after an acute episode of DKA, the recovery of β cell functional reserve was confirmed. This result allowed discontinuation of insulin therapy and maintenance of euglycemic status without antidiabetic medication.