Study Design, A cohort study of 12,000 persons with a follow-up period ranging from birth to the age of 28 years. Objectives, To determine the cumulative incidence of lumbar disc diseases leading to hospitalization in a well-defined geographic population, with special emphasis on age of onset and differences between the genders and among disease categories. Summary of Background Data. There are few published reports on low back pain in adolescence, and most of them are cross-sectional in design. Methods. The Finnish National Hospital Discharge Register was used to identify all possible cases of lumbar disc disease. Copies of all hospital records regarding the diagnosis and course of the disease were collected and the hospitalizations were classified into three categories: I)confirmed herniated nucleus pulposus (HNP), 2) unconfirmed HNP (sciatica symptoms without visible herniation), and 3) other low back diseases. The,:patients were classified according to the highest category Of diagnosis achieved by each. Results. Symptomatic low back pain leading to hospitalization first appeared around the age of 15 years, and the incidence rose more sharply from the age of 19, especially in men with other low back diseases. Men were hospitalized more than twice as often as women. Conclusions. Lumbar disc diseases leading to hospitalization begin early.