Introduction: During the last decade, a number of population-based diabetes registers have emerged which have enhanced the population-based epidemiology of diabetes. The aim of this paper is to review research based on Danish diabetes registers and to compare with similar research in Finland, Sweden, Scotland, and Canada. Research topics: The pattern with the highest prevalences in ages around 75 years is consistent between studies based on different registers, and so is the finding that incidence rates of diabetes are higher among females than males only in ages 20-40. Diabetes registers have been and is increasingly being used to study and particularly quantify links with cardiovascular disease and with cancer. Recently, available medication profiles of diabetes patients have been used as well to further elucidate these links. Conclusion: Diabetes registers are valuable sources of data for description of the trends in occurrence, development, and mortality of diabetes. However, it requires careful application of modern statistical methods since effects of calendar time, age, and duration of diabetes all have to be taken into account when reporting results.