The aim of this study was to obtain an accurate estimate of diabetes prevalence in Croatia and additional estimates of impaired fasting glucose (IFG), undiagnosed diabetes, and insulin resistance. The study was part of the First Croatian Health Project. Field work included a questionnaire, anthropological measurements, and blood sampling. A nationally representative sample of 1653 subjects aged 18-65 years was analyzed. A total of 100 participants with diabetes were detected, among them 42 with previously unknown diabetes. The prevalence was 6.1% (95% CI: 4.59-7.64), with a significant difference by age. IFG prevalence (WHO-criteria) was 11.3%. The ratio of undiagnosed/diagnosed diabetes was 72/100, unevenly distributed by the regions. HOMA-IR was >1 in 40.4% of the subjects. This survey revealed a higher prevalence of diabetes than previously estimated, whereas that of IFG was as expected. A significant difference in the proportion of undiagnosed diabetes among the regions warrants attention.