Autoantibodies to glutamic acid decarboxylase (GADA)-found frequently before the onset of IDDM are an important marker of the autoimmune process of B cells destruction. The aim of the present study was to evaluate the possible relationship between the frequency of GADA and ICA in first degree relatives of subjects with IDDM. The GADA were determined in 238 first degree relatives of IDDM subjects. In 32 of the investigated subjects GADA were measured second time after 12 months and in 24 of them IVGTT was performed to evaluate FPIR and GADA prevalence relationship. The results demonstrate that combination of ICA and GADA antibodies has a high sensitivity and increases the specificity for predicting IDDM in high risk siblings.