While type 2 diabetic subjects in developed countries are predominantly obese or overweight, those in India are often nonobese or lean. The reasons for leanness in these subjects has not been well understood. We assessed the prevalence of pancreatic islet autoimmunity in 83 lean adult subjects (BMI < 18.5 kg/m(2)) with type 2 diabetes by measuring antibodies to glutamic acid decarboxylase-65 (GAD Abs). Positivity to GAD Ab was present in 21 (25.3%) subjects. In addition, subjects with GAD Ab positivity were younger and had lower beta cell function (homeostasis model assessment, HOMA) as compared to the GAD Ab-negative group. This suggests that the antibody-positive group could have a slowly progressive form of type 1 diabetes.