Studies have shown that children and adolescents with low back pain are also similarly affected when they reach adulthood, thus highlighting the importance of investigating causes of low back pain in school-age children. The study examined low back pain and associated factors in 770 schoolchildren 7 to 17 years of age in a private school in Porto Alegre, Rio Grande do Sul State, Brazil. Low back pain was defined as pain or discomfort in the lumbar region in the previous month, assessed by a questionnaire. Low back pain was found in 31.6% of the subjects and was more prevalent in girls (41.9%) than boys (21.4%). Factors associated with lumbar pain were female gender, age 9 to 17 years, hyperactivity (borderline and abnormal categories), and emotional symptoms (abnormal category). Mapping the occurrence of low back pain and associated factors is important for identifying children and adolescents at risk and for developing effective programs for primary prevention.