Objective: In 1999, the World Health Organization (WHO) published new diagnostic criteria for diabetes mellitus (DM). The cut-off value of the fasting plasma glucose concentration was lowered from 7.8 to 7.0 mmol/l. The WHO criteria were used to validate the diagnosis made by the general practitioner, and to compare the diagnostic validity of diabetes mellitus in different countries. Methods: We retrospectively analysed 2556 newly diagnosed diabetics. Incidence was calculated according to the 1999 WHO criteria. Data were collected in general practice networks in five European countries or regions (Belgium, England, the Netherlands, Portugal, Spain). Results: According to the WHO criteria, 82% of the cases were valid diagnoses. Compared to the total group, in Spain, significantly more diagnoses were in agreement with the WHO criteria, whereas this number was significantly lower in England and Portugal. From the patients whose diagnosis was not in agreement with the WHO criteria, significantly more were women than men. Conclusion: By using the WHO diagnostic criteria, the international standard, as a validation tool, we show that the diagnoses of diabetes mellitus made in primary care are valid. Furthermore, we show that these diagnoses are comparable between countries. Therefore, information from general practice registration networks is a valuable and valid source for international comparisons.