Objective: he main objective was to seek, based on defined groups of diabetics, C-peptide levels on fasting and after stimulation which would help to differentiate diabetes mellitus type 1 from diabetes mellitus type 2 in patients with manifestation of diabetes in adult age. Groups: Group A comprised 65 non-obese diabetics type 2 with failure of PAD treatment. Group B included 304 newly manifested diabetics type 1 and 2 aged 31-65 years. Group C was formed by 424 patients with diabetes mellitus type 1 and type 2 with different duration of diabetes. Results: Group A: mean C-peptide levels on fasting 0.32 and after stimulation with a standard breakfast 0.59 pmol/ml suggest absolute insulin deficiency in type 2 diabetics with failure of PAD treatment. Group B: 29.2% diabetics type 1 had already during manifestation of diabetes C-peptide levels on fasting < 0.43 pmol/ml and 47.9 % C-peptide of < 0.6 after a meal. here were 1.9 and 4.9 % subjects among type 2 diabetics with such low C-peptide levels. After a six-year follow up the mean C-peptide levels on fasting declined in type 1 diabetics from 0.49 to 0.16 pmol/ml and in patients originally with type 2 diabetes reclassified to type 1 the levels dropped from 0.56 to 0.26 pmol/ml. Group C served as the basic group for statistically (linear regression method) detected discrimination values of C-peptide differentiating diabetes mellitus type 1 and diabetes mellitus type 2 - the liminal value being 0.59 pmol/ml on fasting and 1.0 pmol/ml after a meal. Conclusion: In clinical practice it is not possible to assess reliably slowly manifesting diabetes type 1 (LADA by age, BMI and compensation of diabetes. Positivity of antiGAD antibodies does not rule out diabetes melli-tus type 1. In unequivocal cases the decisive factor is therefore the C-peptide level on fasting and after a meal.

        Key words: C-peptide - Diabetes mellitus type 1 - LADA - AntiGAD antibodies