Chronic conditions are the major cause of illness, disability and death. A disease register is effective in supporting new models for delivering chronic care. To improve the care of diabetes two stand‐alone diabetes registers have been recently constructed in Finland – a national diabetes register (FinDM II) and a regional diabetes register (T2DR) in the Helsinki metropolitan region. Both compile information from multiple, but separate, databases and could be therefore validated by comparing them with each other.A total of 38 898 and 37 611 diabetic persons were identified from Helsinki and Espoo in the national and regional register, respectively. The numbers were very well matched in the youngest (0‐19 years) and oldest (over 95 years) age groups; in Espoo the match was good also for persons aged 20‐40 years. There were significant differences in the numbers of diabetic persons aged 20 to 65 years; over 3 800 more diabetic persons were retrieved in the FinDM II Helsinki data than in the T2DR data, whereas the T2DR identified 3 100 more senior citizens over the age of 65 years with diabetes than the FinDM II. The possible reasons and implications of these findings to the validity of the registers are discussed
