Objective. Cross-sectional studies have suggested that children who were less physically active and children who watched more television (TV) had more excess body weight, but no large nationwide longitudinal studies have addressed whether children who change their personal levels of activity or inactivity, from one year to the next, experience changes in adiposity. Our objective is to study the association between change in body mass index (BMI) over 1 year and same year change in recreational physical activity and change in recreational inactivity (TV/videos/video games). Design. Cohort study using data from 2 mailed questionnaires, 1 year apart. Participants. A total of 11 887 boys and girls, aged 10 to 15, who returned questionnaires in both 1997 and 1998 as part of the Growing Up Today Study. Outcome Measure. Change in BMI from 1997-1998, accounting for increases in BMI associated with growth and development. Results. An increase in physical activity from 1997 1998 was associated with decreasing relative BMI in girls (-0.06 kg/m(2) per hour increase in daily activity; 95% confidence interval : -0.11, -0.01) and in overweight boys (-0.22 kg/m(2); CI: -0.33, -0.10). An increase in inactivity was associated with increasing BMI in girls (-0.05 kg/m(2) per hour increase in daily TV/videos/video games; CI: +0.02, +0.08). Effects were generally stronger among overweight children. Increasing time doing aerobics/dancing and walking were associated with BMI declines for some groups of children. Conclusions. Many children may benefit by increasing their physical activity and by reducing time watching TV or videos and playing video games. In particular, 2 activities accessible to most children, aerobics/dancing and walking, also appeared beneficial.