Objective:The aim of the study was to investigate the crude prevalence and estimated probability of undiagnosed diabetes in different ethnic groups, given the same level of obesity.Design and Subjects:Cross-sectional data from 24 515 men and 29 952 women, aged >/=30 years, and free of previously diagnosed diabetes were included. Baseline body mass index (BMI) and waist circumference were measured. Diabetes was defined according to both fasting and 2-h 75-g glucose criteria.Results:Prevalence of undiagnosed diabetes was the highest in Asian Indians, the lowest in Europeans and intermediate in others, given the same BMI or waist circumference category across the BMI or waist circumference ranges (P<0.001 for all BMI or waist categories). beta-Coefficients corresponding to a 1 s.d. increase in BMI were 0.34/0.28, 0.41/0.43, 0.42/0.61, 0.36/0.59 and 0.33/0.49 for the Asian Indians, Chinese, Japanese, Mauritian Indians and European men/women (homogeneity test: P>0.05 in men and P<0.001 in women), and in waist: 0.31/0.31, 0.30/0.46, 0.22/0.57 and 0.38/0.58 for the Asian Indians, Chinese, Mauritian Indians and Europeans, respectively (homogeneity test: P>0.05 in men and P<0.001 in women).Conclusion:Prevalence of undiagnosed diabetes increased with an increasing BMI or waist circumference to a similar degree in men in all ethnic groups but to a lesser degree in Asian Indian women than in others, regardless of the higher prevalence in Asian Indians than in others at the same BMI (or) waist circumference levels.International Journal of Obesity advance online publication, 3 November 2009; doi:10.1038/ijo.2009.225.