Background: Type 2 diabetes can progress silently for several years and lead to serious complications. In France, estimates of the prevalence of undiagnosed diabetes are imprecise and are thought to affect 2.0 to 3.7% of the population aged 35 to 65. However, venous blood glucose testing is a simple procedure that can detect the disease at an early stage. Methods: Using data from the Permanent Sample of Socially Insured Individuals from the National Health Insurance Fund for Salaried Workers, which covers 70% of the French population, we selected individuals who had received reimbursement for medical care and estimated the proportion of those who had received reimbursement for a venous blood glucose test or hospitalization, and thus potentially for a blood glucose test, in 2000 or 2001. Results: This rate of opportunistic screening over two years reached 48.6%, representing nearly half of the population that received medical care. Across all age groups, this rate was higher among women than among men. The rate of opportunistic screening increased with age and reached 71.2% among those over 45 years of age. Conclusion: Opportunistic screening for diabetes is therefore commonly performed in France, despite the absence of official recommendations. Several hypotheses may explain the discrepancy between frequent opportunistic screening and the high prevalence of undiagnosed diabetes: 1) undiagnosed diabetics are predominantly found among the population not seeking medical care; 2) opportunistic screening is not performed among those at highest risk; 3) blood glucose tests are not performed on an empty stomach or the results are ignored; 4) management of abnormal blood glucose levels is inadequate; 5) the prevalence of undiagnosed diabetes is lower than current estimates, which are based on a single biological measurement and self-reporting.