The study was undertaken to evaluate (a) the reliability of pedometer data and reactivity of children to wearing a pedometer, (b) the effectiveness of a missing data replacement procedure, and (c) the validity of the Leisure Time Exercise Questionnaire (LTEQ). Six days of pedometer data were collected from 299 middle-school children, followed by administration of the LTEQ. Six days of pedometer data were found to be adequately reliable for research into habitual physical activity (R xx = .79) and no reactivity occurred. Inclusion of weekday and weekend scores is recommended where possible. The individual-centered data-replacement procedure did not adversely affect reliability, so this data-replacement method offers great promise to physical activity researchers who wish to maintain statistical power in their studies. The LTEQ does not appear to measure physical activity similarly to pedometers (r = .05), and researchers should use the LTEQ with caution in children until further research explains this discrepancy.