Factors associated with low back pain (LBP) were sought by means of a questionnaire and physical measurements in 138 adolescents. One-hundred athletes and 38 nonathletes, 138 subjects total (58 boys and 80 girls; age range 10.3-13.3 yr), were studied. The questionnaire included questions on physical activity and LBP. Quantitative measurements on anthropometry, flexibility (joint hypermobility and muscular tightness), and strength were performed. There was no significant difference in the occurrence of LBP between athletes and nonathletes. Among the athletes, the duration of training during the past 12 months (min.wk-1) was higher in subjects with the experience of LBP during the past 12 months (N = 23) compared with nonsymptomatic subjects (N = 76) (mean 493 +/- 308 min.wk-1 vs 354 +/- 160 min.wk-1; P less than 0.0001). Similar differences were also seen between subjects with positive lifetime histories of LBP and nonsymptomatic subjects. Various differences were seen in the measurements of anthropometry, flexibility, and strength between boys and girls as well as between athletes and nonathletes. In a multivariate analysis, the cumulative incidence of lifetime history of LBP was associated with tightness of hip flexor muscles only (P = 0.014). LBP during the past 12 months was associated only with the amount of training during the past 12 months (min.wk-1) (P = 0.0066). Our study suggests that high training duration predisposes young athletes to LBP. However, e.g., the flexibility measurements cannot be used to determine athletes at high risk.