Technical Abstract: To evaluate the impact of a "walking school bus" program on children's rates of active commuting to school and physical activity. We conducted a pilot cluster randomized controlled trial among 4th-graders from 8 schools in Houston, Texas (N = 149). Random allocation to treatment or control conditions was at the school level. Study staff walked with children to and from school up to 5 days/week. Outcomes were measured the week before (time 1) and during weeks 4 and 5 of the intervention (time 2). The main outcome was the weekly rate of active commuting, and a secondary outcome was moderate-to-vigorous physical activity. Covariates included sociodemographics, distance from home to school, neighborhood safety, child BMI z score, parent self-efficacy/outcome expectations, and child self-efficacy for active commuting. A mixed-model repeated measures regression accounted for clustering by school, and stepwise procedures with backward elimination of nonsignificant covariates were used to identify significant predictors. Intervention children increased active commuting (mean +/- SD) from 23.8% +/- 9.2% (time 1) to 54% +/- 9.2% (time 2), whereas control subjects decreased from 40.2% +/- 8.9% (time 1) to 32.6% +/- 8.9% (time 2) (P < .0001). Intervention children increased their minutes of daily moderate-to-vigorous physical activity from 46.6 +/- 4.5 (time 1) to 48.8 +/- 4.5 (time 2), whereas control children decreased from 46.1 +/- 4.3 (time 1) to 41.3 +/- 4.3 (time 2) (P = .029). The program improved children's active commuting to school and daily moderate-to-vigorous physical activity.