Objective: To estimate reporting bias of WebCAAFE, a web-based questionnaire for the assessment of food intake (recall of frequency of intake of thirty-two food items the day before) and physical activity in schoolchildren. Design: Cross-sectional study. Self-reported food intake on WebCAAFE was compared with direct observation of school meals in five public schools. Additional data included school grade, sex, BMI, socio-economic status and access to Internet at home. Poisson regression was used to calculate the reporting bias (WebCAAFE v. direct observation) and the sample size necessary to detect a statistically significant difference between WebCAAFE reports and at least 75 % compliance with the recommendations for a healthy diet. Setting: Intentional sample of five elementary public schools in Florianopolis, Brazil. Subjects: Schoolchildren (n 629) from 2nd to 5th grades. Results: Moderate bias magnitude was found for most food groups of interest. Frequency of consumption was not related to the bias. Sample sizes necessary to detect the compliance with dietary recommendations varied between four and seventy-four individuals for the different groups investigated. Conclusions: After adjusting for moderate bias, WebCAAFE may be used as a food questionnaire for evaluation of schoolchildren's food compliance on a group level, even with a relatively small sample size. The Strategic Action Plan to Tackle Non-Communicable Chronic Diseases in Brazil, 2011Brazil, -2022 , launched by the Ministry of Health in 2010, defined and prioritized actions and investments necessary to address non-communicable diseases in the next 10 years based on three principles: (i) surveillance, information, evaluation and monitoring; (ii) health promotion; and (iii) integral care. The aim is to diminish obesity rates in 5-9-year-old children and adolescents and to halt the rise of obesity in adults (≥18 years old) (1,2) . In Brazil, there are surveillance systems for adolescents (3) and adults (4) but none for 5-9-year-old-children whose overweight (including obesity) prevalence tripled between 1974 and 2009, from 11 % to over 33 % (5) . Therefore it is necessary to develop surveillance and monitoring systems to determine compliance with the obesity prevalence targets for 5-9-year-old children, set at 8 % for boys and 5 % for girls (2) .Periodic population-based surveys are the principal source of time trends in nutritional status and associated lifestyle factors such as diet and physical activity. Monitoring these parameters over the past decades has shown a dramatic increase in excess weight worldwide, especially in younger generations and children (6)(7)(8) . In many countries, the number and complexity of survey items covering these issues have also increased, incurring higher costs (9) . To strike the right balance between the rising costs and a growing need to update our knowledge on population diet and physical activity, it is of utmost importance to clarify what sort of decision could be made on the basis of such surveys an...