This paper describes adolescent low back pain and tests its associations with environmental features of backpack load, time spent carrying loads, time sitting, and time playing sport, using data from 1269 adolescents in twelve volunteer high schools in Adelaide, South Australia. Backpacks were the preferred method of load carrying, two-thirds of wearers preferring to carry the load over two shoulders. The average load weighed 5.3 kg (approximating 10% of body weight). The youngest students carried approximately the same amount as the oldest students. Girls were more likely than boys to report recent low back pain, and there were gender- and age-specific associations between recent low back pain, the amount of time spent sitting, the backpack load and time spent carrying it, and time playing sport. Body mass was not a confounder of any association. These findings support ongoing concerns regarding environmental contributions to adolescent low back pain.