BACKGROUND AND OBJECTIVES: BMI is the most widely used measure to diagnose obesity but its accuracy and usefulness in Saudi subjects is unknown. This study aimed to assess the validity of standard BMI cut-point values in the Saudi population. SUBJECTS/ METHODS: 197,681 adults participated in a cross-sectional study to detect diabetes and hypertension in the Saudi Eastern province in 2004/5, with blood pressure, fasting blood sugar, height and weight measurements taken. Sensitivities, specificities, areas under the curves, predictive values, likelihood ratios, false positive, false negatives and total misclassification ratios were calculated for various BMI values determined from receiver operating characteristic (ROC) curves. The significance of association between risk factors and BMI was assessed using regression analysis. RESULTS: For the definition of overweight, ROC curve analysis suggested optimal BMI cut-offs of 28.50 to 29.50 in men and 30.50 to 31.50 in women, but the levels of sensitivity and specificity were too low to be of clinical value and the overall misclassification was unacceptably high across all the selected BMI values (> 0.80). The relationship between BMI and presence of diabetes and/or hypertension was not improved when a BMI of 25 was used. Using regression analyses, the odds ratios for hypertension and/or diabetes increased significantly from BMI values as low as 21-23 with no improvement in the diagnostic performance of BMI at these cutoffs. CONCLUSION: In Saudi population, there is an increased risk of diabetes and hypertension relative to BMI, starting at a BMI as low as 21, but overall there is no cut-off level of BMI with high predictive value for developing these chronic diseases, including the WHO definition of obesity of 30.