Objective:To determine the prevalence of low back pain in children and its relationship with gender, age, exercise practice and sedentary activities (time on TV and on computer and/or video game).Methods:We conducted a cross-sectional study with a sample of 1,461 students, being 754 males and 707 females enrolled in the 5th to 8thgrades of all five local elementary schools of urban Ourinhos, SP, Brasil, aged 10-14 years old, from whom sociodemographic data, time spent on sedentary activities such as TV and computer and/or video game were collected. We conducted a descriptive, bivariate and multivariate binary logistic regression.Results:The results showed that the prevalence of low back pain in the sample was 18.5%, being 15.5% in boys and 21.6% in girls. Low back pain was associated to the female gender, age range between 12-14 years and the habit of watching TV more than three times a week and over 3 hours per day.Conclusion:The students analyzed showed high prevalence of low back pain related to gender, age and habit of watching TV. Appropriate interventions should be implemented to reduce the occurrences of back pain in schoolchildren.Level of Evidence III, Cross Sectioning.