Our aim was to determine the prevalence of diabetes mellitus in the 25-to-74-year-old population in the province of Gerona, Spain. History of known diabetes mellitus was recorded, and fasting glycemia was measured in venous blood. The 1997 diagnostic criteria of the American Diabetes Association were used. Crude prevalence of known diabetes mellitus was 10.0%, and age-standardized prevalence was 7.7% (95% confidence interval [CI], 7.3%-8.1%). Crude prevalence of impaired fasting glucose was 8.6%, and age-standardized prevalence was 7.6% (95% CI, 7.25%-8.1%). Crude prevalence of known diabetes mellitus combined with diabetes mellitus according to glycemia value (total prevalence of diabetes mellitus) was 13.0%, and age-standardized prevalence was 10.0% (95% CI, 9.6%-10.5%). A higher prevalence in men and an increase in prevalence with age were observed. The figures are different from those of other studies in Spain.