PURPOSE: To evaluate the effect of a 17-week intervention, including an after-school physical activity (PA) club 3 d/wk, on moderate-to-vigorous physical activity (MVPA), body mass index (BMI) z score, percentage body fat (%BF), and cardiorespiratory fitness (CRF) among fifth to eighth grade girls having a BMI z score >= 0, and explore whether intervention outcomes varied by club attendance (1 versus 2 versus 3 d/wk). DESIGN: Secondary analysis of data from a group randomized controlled trial (N = 1,519, 10- to 15-year-old girls: n = 753 intervention; n = 766 control). SETTING: Twenty-four Midwestern US schools (n = 12 intervention; n = 12 control). SAMPLE: Subsample (n = 1,194 girls) from trial's intervention (n = 593 girls) and control (n = 601 girls) groups having BMI z scores >= 0. MEASURES: Moderate-to-vigorous physical activity (min/h), BMI z score, %BF, and CRF (VO2max: mL/kg/min) were estimated at baseline and postintervention. ANALYSIS: Linear mixed-effect models. RESULTS: Intervention group gained less %BF (B -0.35, p = 0.016), and their CRF decreased less (B 0.22, p = 0.010) than the control. Marginally significant findings showed girls attending the club an average of 1 d/wk had greater increases in %BF (B 0.33, p = 0.087) and MVPA (B 0.20, p = 0.083) and a greater decrease in CRF (B -0.20, p = 0.061) than girls attending 3 d/wk. No differences occurred between girls who attended 2 versus 3 d/wk for any outcomes. CONCLUSIONS: The intervention attenuated an increase in %BF and a decrease in CRF among girls at risk for obesity from baseline to postintervention. Offering the after-school PA club 2 d/wk may be adequate for achieving outcomes.

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