Introduction: Maturity-onset diabetes of the young (MODY) is a monogenic form of diabetes that is characterized by an early onset (usually before 25 years), autosomal dominant mode of inheritance and a primary defect in pancreatic β-cell function. MODY is a common form of monogenic diabetes and it may account for 1% to 2% of all diabetes cases in Europe. Many people with MODY are misdiagnosed with type 1 or type 2 diabetes. Materials and Methods: All adult subjects with diabetes onset under age of 45 years and currently older than 18 years. Other inclusion criteria are: evidence of endogenous insulin secretion (fasting or random C-peptide ≥ 0.2 nmol/L) and negative glutamic acid decarboxylase antibodies (GADA), islet cell autoantibodies (ICA) and islet tyrosine phosphatase 2 (IA2). We analysed 44 patients for HNF1A-MODY, 14 patients for HNF4A-MODY and 8 patients for HNF1B-MODY. For identification of mutations in the coding and promoter region of analyzed genes, we used the Sanger sequencing method. Results: We identified six different mutations in nine patients for subtype HNF1A-MODY. These mutations are located in exons 2, 3, 4 and 6, but the greatest number are found in exon 4, p.Gly292Argfs*25. For subtype HNF1B-MODY we identified one mutation, p.Val458Gly in exon 7. No HNF4A-MODY gene mutations were identified in any of patients analyzed. Conclusions: Correctly identifying MODY has important implications for treatment, surveillance of complications and associated extra-pancreatic disorders, and identification of affected and at- risk family members.