Clinical symptoms of type 1 diabetes are preceded by a long period of prediabetes stage characterised by anti-islet antibodies occurrence as well as insulin and C-peptide secretion disturbances. The aim of this study was to define the prognostic value of type 1 diabetes antiislet humoral markers (ICA, anti-GAD, anti-IA2 and IAA) and to find out thresholds for insulin and C-peptide levels at which clinically overt type 1 diabetes develops. Antiislet antibodies, serum C-peptide and insulin were determined in 86 children who, considering their antiislet autoantibodies levels, were classified as prediabetics (mean value of the observation period: 50 months). 8 (9.3%) children, who after a mean time of 35 months of prediabetes stage developed clinically overt type 1 diabetes, were selected from this group. ICA were determined by indirect immunofluorescence; anti-GAD and IAA by radioimmunoprecipitation. C-peptide and insulin levels were evaluated by radioimmunologic assays (CIS Bio International, France). Kaplan-Meier life table analysis revealed pEFS=0.89 after 92 months' observation. The risk of developing diabetes within 80 months was established. For children with positive ICA the risk rate was 0.21, for ICA and anti-GAD positive individuals - 0.39, and for ICA and IA2 positive - 0.74. A significant difference in insulin and C-peptide levels was found between children who developed clinically overt type 1 diabetes and those in prediabetes stage (9.90 vs. 21.45 micro U/ml, p<0.008; 0.34 vs. 0.67 pM/ml, p<0.001 respectively). For both hormones thresholds for high risk of developing clinically overt diabetes were pointed out. Using ROC method the threshold for insulin was determined at 12.9 micro U/ml, for C-peptide at 0.45 pM/ml. Not only the presence and levels of autoantibodies but also the plasma concentrations of C-peptide and insulin are important prognostics of clinical onset of type 1 diabetes mellitus.