To clarify the utility of islet cell antibodies (ICA) to correctly classify and predict insulin treatment in newly diagnosed diabetic subjects, ICA, body mass index (BMI), glycated hemoglobin (HbA1c), and fasting plasma C-peptide values were evaluated at and 3 years after diagnosis in 233 new, consecutively diagnosed, adult diabetic patients classified as obese or nonobese (National Diabetes Data Group, NDDG criteria). Among the 233 patients, 31 were nonobese ICA-positive (mean age at diagnosis 43 +/- 3 years), 55 nonobese ICA-negative (mean age at diagnosis 58 +/- 2 years), 7 obese ICA-positive (mean age at diagnosis 57 +/- 5 years), and 139 obese ICA-negative (mean age at diagnosis 58 +/- 1 years). Fasting C-peptide decreased (P < 0.05) in nonobese ICA-positive patients who after 3 years showed lower BMI (22.6 +/- 0.6 versus 24.5 +/- 0.4; P < 0.05), lower fasting C-peptide (0.14 +/- 0.06 nmol/l versus 0.71 +/- 0.07 nmol/l; P < 0.001), and higher frequency of insulin treatment [28/31 (90%) versus 6/45 (13%); P < 0.001] than nonobese ICA-negative patients.(ABSTRACT TRUNCATED AT 250 WORDS)