OBJECTIVE To measure the prevalence of children with adequate weight who feel fat and to examine the factors associated with this perception. METHODS Cross-sectional study with 901 schoolchildren aged 8-11 years selected by cluster sampling. The children had their weight and height measured, and answered a questionnaire that included a self-esteem scale and questions on self-perception of weight, and perception of parents' and friends' expectations regarding the child's weight. RESULTS The prevalence of children with BMI percentile < 85 who considered themselves fat was 13%, and the variables significantly associated with this perception were: female gender (OR = 2.45; 95%CI 1.42-4.24), 11 years of age (OR = 2.35; 95%CI 1.13-4.89), lowest quartile of self-esteem (OR = 2.08; 95%CI 1.17-3.68), the perception that parents expect them to be thinner (OR = 3.00; 95%IC 1.52-5.91), and body mass index percentile (OR = 1.04; 95%CI 1.03-1.06). CONCLUSIONS The perception of being fat when having adequate weight afflicts children before preadolescence, particularly girls aged 11 years, with higher body mass index, lower self-esteem, and who think their parents expect them to be thinner. Future studies should examine in depth the causes and consequences of this attitude.