This study was designed as a cross-sectional questionnaire-based survey of low back pain (LBP) in 13- to 16-year-old Danish school children. The cohort consisted of 671 boys and 718 girls in eighth and ninth grade in 46 municipal schools in three counties of Sealand. All the pupils filled in a questionnaire with LBP as the main topic and were at the same time examined by the school doctors. The first part of the questionnaire contained questions about leisure time sports activity, TV watching, PC use, job in leisure time and smoking. The second part dealt with LBP in relation to frequency and severity, influence on daily living and use of the health system. The school doctor measured body height and weight, (BMI), degree of hypermobility and the tightness of the hamstring muscles. The results showed a cumulative life-time prevalence of LBP of 58.9%, a 1-year prevalence of 50.8% and an increase in LBP prevalence of 6.4% from 14 to 15 years of age, independent of gender. Fourteen percent (141 F, 54 M) fulfilled the criteria for general hypermobility and 12.2% (45 F, 124 M) had tightness of hamstring muscles of more than 40 degrees. Recurrent/continuous LBP in a moderate to severe degree was recorded in 19.4% of children (182 F, 88 M). This was positively correlated to female gender, BMI more than 25 kg/m(2), competitive sport for boys, poor physical fitness, daily smoking, heavy jobs in leisure time, increased use of the health system and reduced life quality. Stepwise logistic regression analysis indicates that female gender, daily smoking and heavy jobs are important associated factors for severe LBP in adolescents, with an observed probability of 46% if all factors are present. We don't know yet whether these factors are of any causal importance in the development of severe LBP.