Background: To present actual data to estimate prevalence, incidence and mortality of known type 2 diabetes mellitus in all age categories in The Netherlands. Methods: Prospective population-based study between 1998 and 2000 in The Netherlands. Baseline population of 155,774 patients, registered with 61 general practitioners participating in the Zwolle Outpatient Diabetes project Integrating Available Care ( ZODIAC)-study. Results: Age- and sex-adjusted prevalence of type 2 diabetes was 2.2% at baseline and 2.9% after 2 years of follow-up; for women and men it was 3.1 and 2.7% at follow-up, respectively. Patients aged > 70 years account for almost 50% of all type 2 diabetes patients. Age- and sex-adjusted mean annual incidence per 10,000 over 3 years was 22.7 overall; for women 23.1 and for men 22.2. Incidence - even though high - decreases after the age of 70 years. The mortality rate was 47.9/1000 and standardised mortality ratio 1.40. Based on these results, the estimated total number of subjects known with type 2 diabetes was 466,000 for The Netherlands in 2000; the number of patients with newly diagnosed diabetes 36,000. Conclusions: Prevalence and incidence rates exceed all estimates regarding known type 2 diabetes for The Netherlands. Elderly patients, aged 70 years and over, account for 50% of the type 2 diabetic population. These results are important for health-care planning.