The purpose of this study was to determine: (i) the effect of schools' baseline community readiness (CR) on youth physical activity (PA) at recess prior to the Ready for Recess intervention; (ii) if changes in PA due to the intervention were explained by baseline CR and (iii) if specific components of the intervention altered an association between baseline CR and changes in youth PA. Methods that were employed included: six informants from each of 17 schools participated in CR interviews at baseline (N = 101). CR scores were calculated based on the CR model's nine stages. Direct observation was used to measure PA. Poisson models evaluated the association between baseline CR and PA. Results were that seven schools were in denial, eight in vague awareness and two in pre-planning stages. CR marginally predicted pre-intervention PA. When compared with youth at schools in denial, youth at schools in vague awareness and pre-planning demonstrated significant increases in moderate/vigorous PA. Ready for Recess strengthened this association. This is the first study to demonstrate that school CR may explain changes in PA at recess after a school-based PA intervention. Low CR levels may contraindicate interventions. Efforts to increase school CR "a priori" may be critical to increasing PA among youth.