For this study, subjects, ages 12 to 18 years, were recruited from 12 middle schools and 10 high schools. The schools differed in physical layout (single building or campus-like facility with multiple buildings), daytime backpack use policy (permitted or prohibited), and type (public or private). This cross-sectional analysis was derived from the baseline survey of an ongoing longitudinal prospective study and was conducted between November 2000 and May 2001