AIMS: Comparing waist circumference (WC) role in diabetes risk prediction and diagnosis of metabolic syndrome (MS) in different populations. METHODS: Population-based samples from Australia (n=9026) and Iran (n=8259) were studied in 2000 and followed for approximately 4 years. Follow-up attendance was approximately 58% and mean age was 51 vs. 47. Pearson correlations calculated between WC and other MS components. ROC for the role of WC in the prediction of incident diabetes was used. RESULTS: Prevalences of MS (48% vs. 28%), an increased WC (58.5% vs. 54.5%), low HDL-C (35% vs. 11.2%), high triglyceride (52.2% vs. 29.6%) were significantly higher in Iran. Fasting glucose >or=5.6mmol/L was higher in Australia (26% vs. 23%). Hypertension was no different ( approximately 38%). Pearson correlations between WC and other MS components were stronger in Australians: FPG (0.32 vs. 0.2), HDL (0.47 vs. 0.16), TG (0.38 vs. 0.30) and SBP (0.38 vs. 0.36). Among women, area under ROC curve for WC as a predictor for diabetes was significantly higher for Australians (0.76 vs. 0.68, p<0.001) with no difference among men (0.69 vs. 0.71, p=0.4). CONCLUSION: WC was more strongly related to other components of MS in Australia. Association between WC and MS or incident diabetes varies between ethnicities.