Haemoglobin A(1)c (HbA(1)c) or glycohaemoglobin is one of the most important parameters in the management of patients with diabetes mellitus, but to date there is no international standard for determining HbA(1)c. Most of the routine HbA(1)c assays are standardised against one of the local standardisation schemes like the NGSP (USA) and other schemes (Japan, Sweden). Still, results of HbA(1)c tests diverge considerably, as do the accompanying clinical decision limits. The IFCC Working Group on HbA(1)c Standardisation has developed a reference method and also set up a reference system for HbA(1)c, in which the analyte is defined as beta-N-glycated haemoglobin. This reference system consists of a network of reference laboratories that uses the reference methods and certified reference materials for optimal measurement of HbA(1)c in human blood. The main task of the network is to assign values to secondary reference materials, to be used by manufacturers of routine HbA(1)c assays to calibrate their assays. The high specificity of the reference method results in lower HbA(1)c values in blood samples, since the unspecific components falsely identified as HbA(1)c in routine methods are not measured by the reference method. The reference range for the new reference method was determined as 3 to 4% and the clinical decision limits were translated from existing guidelines: goal of treatment 5% HbA(1)c, change of therapy advised at HbA(1)c greater than 6%. Despite these lower values, worldwide implementation of the IFCC reference system for HbA(1)c is recommended, in order to end the great divergence in HbA(1)c results, with which physicians and patients are confronted today.