From a population of 1,503 schoolchildren, 38 15-year-old children reporting recurrent or continuous low-back pain and 38 asymptomatic controls (34 boys and 42 girls) matched for age, sex, and school class were selected for tests of spinal mobility and trunk muscle strength, and for a magnetic resonance imaging (MRI) study of the lumbar spine. In addition, the subjects were asked about leisure time physical activities in an interview preceding the measurements. Spinal muscular atrophy was the only finding that was more common among physically inactive subjects (p = 0.005). Moreover, increased occurrence of disk degeneration (DD) was observed in the low-activity group. However, the difference was not significant. Neither Scheuermann-type changes nor DD were related to spinal mobility or trunk muscle strength. Children with disk protrusion were, on the average, taller (p = 0.044), and their lumbar flexion measured by flexicurve was decreased (p = 0.043). Our results strengthen further the evidence that MRI is a sensitive measure and that imaging findings must be interpreted carefully with respect to pain and physical impairment of the lumbar spine. Furthermore, no clear evidence of the association between physical activity and early DD could be found.