The study aims to describe the influence of sports activities, high body weight and smoking on low-back pain. The results were obtained from a population-based case-referent study, the Musculoskeletal Intervention Center (MUSIC)-Norrtälje study. In all, 342 male and 449 female cases, and 662 male and 948 female referents participated. Neither low-intensity training many hours/week (> or = 5 h) nor high-intensity training few (1-2 h), intermediate (3-4 h) or many hours (> or = 5 h) per week affected the risk of low-back pain among men. Few (1-2) hours with high-intensity training increased the relative risk of low-back pain among women, RR 1.6 (1.1-2.4). An increased risk of low-back pain was found for men with high body weight, RR 2.2 (CI 1.2-3.9) but not for women. Smoking did not influence the risk of low-back pain.