Background: Hypertension in children is a frequently overlooked problem that is an important cardiovascular risk factor. Aim: To determine the prevalence of hypertension among school age children. Material and Methods: Cross-sectional study of 2980 children aged 10 ± 2years (48% females) from 10 schools of middle and lower class in Metropolitan Santiago. Blood pressure (BP) was measured in the sitting position on three occasions after a rest period, using a mercury sphygmomanometer with appropriate cuff arm diameter, averaging the results of the measurements. Systolic and diastolic hypertension were defined as blood pressure values over 95percentilefor age, sex and height. Results: The overall prevalence of hypertension was 12.2% in women and 15% in men (p < 0.05). According to nutritional status, the prevalence was 6.7, 8.9,13.6 and 26% in underweight, eutrophic, overweight and obese children, respectively (p < 0.01). Compared with normal weight children, the risk of being hypertensive for overweight children was 1.6 (95% confidence intervals (CI) 1.2-2.3) and for obese children was 3.6 (95% CI 2.8-4.7). Conclusions: The studied children had a high prevalence of hypertension, that was directly related to a higher body mass index.