Background. Frequency of autoantibodies anti-GAD65 positivity in 2245 diabetics of Type I with different length of disease and the relation of the positivity to other autoantibodies, and to other biochemical and clinical criteria was examined. The aim of the study was to determine the possible effect of anti-GAD65 positivity on the development of Type I diabetes mellitus (DM) and its complications. Methods and Results. Anti-GAD65 positivity was found in 103 Type I diabetics (42.04 %). The highest prevalence of positive results was found among patients with the disease lasting 1 to 5 years (65 %) and also in the group of patients with the disease lasting more than 10 years (24 %). The group of anti-GAD65 positive Type I diabetics had statistically higher levels of anti-IA-2 autoantibodies (15.70 U/ml, SD=38.66), (9.13 U/ml, SD=14.39), anti-TPO (374.99, kU/ml), SD=290.42), and anti-Tg (189.12 kU/ml, SD=690.90) in comparison with diabetics without any anti-GAD65 autoantibodies (0.93 U/ml, SD=2.46), IAA (2.71 kU/ml, SD=6.25), anti-TPO (108.09 kU/ml, SD=171.01), and anti-Tg (77.08 kU/ml, SD=76.00). Diabetics with anti-GAD65 positivity in comparison with those with anti-GAD65 negativity had significantly shorter duration of the disease (6.73 years and 12.9 years respectively), lower BMI (23.70 and 27.00 respectively), and the postprandial C-peptide level (0.27 pmmol/ml and 0.39 pmol/ml respectively). Frequency of positive findings of anti-GAD65 positivity was significantly higher in females than in males. Neither the age of patients in time of DM diagnosis, nor the DM compensation (HbA1c) differs in the studied groups. Conclusions. Anti-GAD65 positivity persists in part of the adult diabetics during long-lasting DM, though it continually decreases with the duration of the disease. Anti-GAD65 positive diabetics are slimmer, their disease is shorter, and they have lower postprandial C-peptide levels and more frequent positivity in anti-IA-2 autoantibodies and in IAA. Incidence of the diabetic retinopathy and microalbuminuria are similar to that in anti-GAD65 negative patients.

        Key words: anti-GAD65, C-peptide, Type I diabetes mellitus.