A longitudinal study was undertaken to analyse the development of posture and spinal mobility during growth and its relationship to low back pain and sports activities. A total of 90 children were examined at 5–6 years of age and re-examined at 15–16. Sagittal configuration and mobility were measured using Debrunner's kyphometer. Information about pain and activities was acquired by interview with the parents of the 5- to 6-year-olds and by a questionnaire to the 15- to 16-year-olds. Posture changed significantly during the study period: thoracic kyphosis increased by 6° and lumbar lordosis increased by 6°. The relationship between kyphosis and lordosis was independent of gender at age 5–6, but kyphosis in relation to lordosis was significantly lower in girls among the 15- to 16-year-olds. The total sagittal mobility of the spine decreased significantly during the 10-year study period: in the thoracic spine by as much as 27° and in the lumbar spine by 4°. About one-third of the children at the age of 15–16 years stated that they had occasional low back pain. This complaint was more frequent in those stating they had suffered some type of back injury, but low back pain was not related to gender, regular physical training, posture or spinal mobility. The results of the study showed that kyphosis and lordosis increased and mobility decreased in the 90 children who were examined both at age 5–6 and 15–16 years. The relationship between kyphosis and lordosis decreased in girls but not in boys. Occasional low back pain was reported by 38% of the children at the age of 15–16 years, but back pain was not related to posture, spinal mobility or physical activity.