AIM: To estimate the prevalence of undiagnosed diabetes and impaired fasting glucose in older British men and women, using the 1999 World Health Organization (WHO) thresholds based on fasting glucose measurements. METHODS: Participants in the British Regional Heart Study and the British Women's Heart and Health Study were selected from one socially representative general practice in 24 British towns. Included in this analysis were 3736 men and 3642 women aged 60-79 years (predominantly white), who provided a single fasting blood sample at a clinical examination between 1998 and 2001, and who had no previous diagnosis of diabetes. RESULTS: Two hundred and eleven men (5.7%) and 190 women (5.2%) had a fasting blood glucose level consistent with the WHO threshold for a diagnosis of diabetes (> or = 7.0 mmol/l), whilst a further 667 men (17.9%) and 642 women (17.6%) had impaired fasting glucose levels (6.1 < or = 7 mmol/l). When analyses were restricted to subjects who had fasted for at least 8 h, and whose blood sample was taken before 12.00 h, the predicted prevalence of undiagnosed diabetes (based on two separate measurements) was 6.7% in men and 6.0% in women. The predicted prevalence of impaired fasting glucose (based on two separate measurements) was approximately 20% in both sexes. CONCLUSIONS: More than one-fifth of older white British men and women have either undiagnosed diabetes or impaired fasting glucose according to new WHO criteria. Strategies for the primary and secondary prevention of Type 2 diabetes among older individuals are urgently needed.