Self-perception of body image is one factor to be considered when planning and performing preventive initiatives and interventions in childhood and adolescence. The aim of this study was to analyse self-perception of body image in relation to body-mass index, physical inactivity, not eating breakfast, dieting, smoking and alcohol consumption or toxic habits. Materials and Methods: using data from the “Quito municipal schools” cohort study 6964 students aged from 9 to 17 years were described using a multivariate log linear model of the multidimensional table generated by the variables. A logistic regression model was then fitted to assess associations via OR. Results: 8.2% perceived themselves as overweight. Of those subjects overweight according to their BMI, 21.8% perceived themselves as such, while among those not overweight according to BMI, 96.8% considered themselves as slim or of normal weight. Among students who were dieting, 15.1% perceived themselves as overweight. Among those pupils who perceived themselves has having excess weight, the most common reasons for dieting were: lose weight (56.8%), be healthier (22.6%), and maintain current weight (8.4%). Self-perception of excess weight interacts with excess weight (OR_adjusted 8.42; CI_95% 6.92-10.25), no breakfast (OR_adjusted 2.83; CI_95% 2.13-3.77), diet (OR_adjusted 2.38; CI_95% 1.95-2.89), and with all the variables except toxic habits (OR_adjusted 1.01; CI_95% 0.78 a 1.29). Conclusion: Interventions to prevent obesity in childhood and adolescence ought to take account of specific determinants within the personal, behavioural and socio-environmental factors, such as the promotion of a positive body image.