Patients with autoimmune type 1 diabetes mellitus have often, besides immune diabetes markers, also other organ-specific antibodies, particularly thyroid autoantibodies (antithyreoglobulin antibodies - ATG and/or thyroid peroxidase antibodies - TPO). In many of these patients autoimmune thyroid diseases, i.e. Hashimoto thyroiditis and Grave's disease, with silent clinical course can be diagnosed. The aim of this study was to evaluate the prevalence of TPO and ATG antibodies in children with newly diagnosed type 1 diabetes treated in two diabetes centres, in Lodz and in Kielce. Elevated ATG and/or TPO antibodies were found in 17,8% (15/84) of children: in 19,2% (11/57) in Lodz centre and in 14,8% (4/27) in Kielce centre. Children with elevated thyroid autoantibodies were significantly older than those without thyroid autoantibodies. Daily insulin requirement did not differ between both groups.