Among the spectrum of different types of diabetes mellitus (DM), latent autoimmune diabetes in adults (LADA) distinguishes itself from the more prevalent type 2 DM (T2DM) by a composite of specific clinical features (onset of diabetes at adult age with non-insulin-therapy as initial regimen, but progresses rapidly to insulin-requiring due to primary failure to oral antidiabetic drugs (OADs) for optimal glycemic control), lower C-peptide levels, and presence of auto-antibodies against antigens derived from pancreatic islets (with antibody against glutamic acid decarboxylase(GAD)-65 having the highest diagnostic accuracy). A correct categorization into LADA at the onset is sometimes challenging because the phenotypic presentations in an adult patient will very likely lead clinicians to prescribing OAD as starting regimen as for patients diagnosed with classic T2DM, especially in the absence of diabetic ketosis/ketoacidosis as initial presentation. Laboratory findings including a low basal (fasting) and/or glucagon-stimulated C-peptide levels can help verify the insulin-dependence state of the patient, and a positive test for GAD-65 antibody may further consolidate the diagnosis of LADA. Early institution of insulin therapy should be considered in order to not only achieve more optimal glycemic control but also preserve and improve β-cell function during the long-term disease course.  (J Intern Med Taiwan 2021; 32: 398-410)