Study Design. A 5-year longitudinal interview and questionnaire-based survey of back pain in adolescents. Objectives. To determine the natural history of back pain during adolescence in boys and girls, and to explore the influence of sports participation and lumbar flexibility. Summary of Background Data. Previous data on low back pain and flexibility in adolescents have come largely from cross-sectional studies with differing definitions and age groups. A longitudinal study would offer a more detailed description of aspects of the natural history of back pain. Methods. A cohort of 216 11-year-old children was given a structured questionnaire about back pain. Follow-up evaluation was annual for 4 more years. Lumbar sagittal mobility was measured in first and last years. Life-table analysis was the chosen statistical method. Results. Annual incidence rose from 11.8% at age 12+ to 21.5% at 15+ years. Lifetime prevalence rose from 11.6% at age 11+ to 50.4% at age 15+ years. Experience of back pain was frequently forgotten. Recurrent pain was common, usually manifesting as such rather than as progression from a single episode; few children required treatment. Back pain was more common in boys than girls, especially by age 15 years. There was a positive link between sports and back pain only for boys. Severity and flexibility were not related to sex, treatment, or sport. Conclusions. Back pain in adolescents is common; it increases with age and is recurrent, but in general does not deteriorate with time. Much of the symptomatology may be considered a normal life experience, probably unrelated to adult disabling trouble.