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Background: Our aim was to combine several regional studies in order to estimate the prevalence of diabetes in Norway., Material and Methods: We used data from a nation-wide registration of type 1 diabetes in the age group < 30 years. Data for other age groups were taken from nine regional surveys, of which six were repeated. The average prevalence of known diabetes based on self-administered questionnaires was estimated. Data on previously undiagnosed diabetes were available from two Norwegian studies and from other countries., Results: The sex and age standardised prevalence of "known" diabetes (all age groups) was estimated at 2.3%, increasing with age to approximately 8% among the 70 to 79-year-olds. The prevalence was 3.4% among those aged > or =30 years. The estimated increase in prevalence of known diabetes over time was 1.4% per calendar year, with variation between sub-groups. The number of unknown cases may be nearly equal to the number of known cases in the age-groups > or =30 years, but this is uncertain., Interpretation: There are probably about 90,000 to 120,000 people with diagnosed diabetes in Norway. Nearly as many may have undiagnosed diabetes, but this is uncertain. Studies representative of the Norwegian population using the oral glucose tolerance test are needed.