To research into the impact of the new American Diabetes Association (ADA) diagnostic criteria on high risk Spanish population, two cross-sectional studies involving seven primary health care centers in Catalonia (Spain) were revised. Individuals aged > 40 years with any major risk factor for diabetes were screened according to the World Health Organization (WHO) rules using a 75 g oral glucose tolerance test to measure fasting plasma glucose (FPG) and 2 h plasma glucose. The changes on diabetes prevalence and on epidemiological characteristics were evaluated applying the ADA criteria on the basis of FPG alone. A total of 970 individuals, 453 males (46.7%), mean age 59 years and mean body mass index (BMI) 30.6 kg/m2 were screened. Among the 459 diabetic subjects according to either the WHO or the ADA criteria, 314 (68.4%) were classified as having diabetes with respect to both sets of criteria (WHO and ADA). The overlap between impaired glucose tolerance (WHO) and impaired fasting glucose (ADA) diagnoses was 20.7%. Using the ADA criteria results in a decrease of the prevalence of diabetes by 1.5% (95% confidence interval (CI) = -2.2 to -0.8%). No changes in the diabetic phenotype (age, sex and BMI) were found. Impaired fasting glucose prevalence was 18.4% (95% CI = 16-21%). Overall concordance in terms of crude and weighted kappa-value was only acceptable (kappa = 0.51 and kappa = 0.61, respectively). To apply the new ADA diagnostic criteria on high risk Spanish population evidenced a decrease on diabetes prevalence. Nevertheless, the change of criteria undervalued the risk of postprandial hyperglycaemia related to impaired glucose tolerance.