OBJECTIVES: To evaluate the effect of urbanization and ethnicity on correlations between waist circumference (WC) and obesity-related cardiovascular risk factors. METHODS: 1471 rural and urban Cameroonians, and 4185 French, from community-based studies, aged > or =25 years, not treated for hypertension, diabetes and dyslipidemia participated in this study. Slopes of obesity-related abnormalities with WC were compared using an interaction term between place of residence and WC. RESULTS: Women in urban Cameroon and men in France had significantly higher WC and BMI relative to their gender counterparts. Urban Cameroonians had higher abdominal adiposity, but lower BP and better metabolic profile than the French. WC was positively associated to all the obesity-related abnormalities in the three sites except to FPG (both genders) and blood lipids (women) in rural Cameroon. A 5 cm larger WC was associated with a higher increment among urban than rural Cameroonians for diastolic blood pressure (DBP) (women, 1.95/0.63 mm Hg; men, 2.56/1.44 mm Hg), HOMA-IR (women, 0.11/0.05), fasting plasma glucose (FPG) (men, 0.09/-0.01 mmol/l) and triglycerides (women, 0.06/0.01 mmol/l; men, 0.09/0.03 mmol/l), all P<0.05. A 5 cm larger WC was associated with a higher increment among urban Cameroon than French people for DBP (women, 1.95/1.28 mm Hg, P<0.01; men, 2.56/1.49 mm Hg, P<0.01), but with a lower increment for HOMA-IR (women, 0.11/0.14, P<0.05), FPG (women, 0.05/0.09 mmol/l), total cholesterol (women, 0.07/0.11 mmol/l; men, 0.10/0.13 mmol/l) and triglycerides (women, 0.06/0.11 mmol/l; men, 0.09/0.13 mmol/l) all P<0.05. CONCLUSION: Ethnicity and urbanization modify the association of WC with obesity-related metabolic abnormalities. WC cutoff points derived from Caucasians may not be appropriate for black Sub-Saharan Africans.