Study Design. This was a prospective 3-year follow-up study of randomized matched subgroups of 15-year-old school children with or without low back pain initially. Objective. In addition to low back pain and leisure time physical activity, spinal mobility, trunk muscle strength, and early degenerative findings of the lumbar spine were evaluated. Summary of Background Data. Reliable epidemiologic studies on the prevalence of low back pain and development of early degenerative changes of the lumbar spine in young persons have been sparse. Along with several other characteristics, the relationship of these changes to frequent low back pain in young persons is not known. Methods. After a questionnaire-based survey was administered, subjects with or without low back pain were examined initially and at follow-up with special reference to leisure time physical activity, anthropometry, spinal mobility, trunk muscle strength, and magnetic resonance imaging findings of the lumbar spine. Results. At baseline and at follow-up, those subjects with initial low back pain were characterized by a low frequency of physical activity and decreased spinal function. During follow-up, the occurrence of disc degeneration increased significantly more in the original group with low back pain than among asymptomatic subjects. Furthermore, disc degeneration at baseline significantly predicted future frequent low back pain. Initial disc protrusion also predicted future frequent low back pain. Conclusion. After the rapid physical growth period, there seemed to be a causal relationship between the early evolution of the degenerative processes of the lower lumbar discs and frequent low back pain in several subjects.