Introduction: This study evaluated whether intervening with child care providers would increase physical activity (PA) and reduce adiposity in preschoolers. Methods: This was a two-arm, parallel group, cluster RCT whereby six child care centers were randomly assigned in equal numbers to intervention (n=40 children) or control (n=43 children). Participants were aged 3-5 years and attended licensed child care centers. Child care providers received two 3-hour train-the-trainer workshops and a training manual at program initiation aimed at increasing structured and unstructured PA through active play. Control child care centers implemented their standard curriculum. PA and sedentary behavior were measured by accelerometry, and body composition was measured using bioelectrical impedance at baseline and 6 months. Data were collected in 2011-2012 and analyzed in April 2015. Results: Linear mixed-effects modeling showed that at 6 months, children in the intervention group demonstrated greater increases in minutes per preschool day spent in overall PA (22.5 minutes, 95% CI=8.9, 36.1, p=0.002), and light-intensity PA (16.1 minutes, 95% CI=5.2, 26.7, p=0.004), but changes between groups in moderate to vigorous PA did not differ. The intervention group showed greater reductions in body fat percentage (-1.9%, 95% CI=-3.5, -0.3, p=0.023) and fat mass (-0.3 kg, 95% CI=-0.7, -0.1, p=0.018), but groups did not differ on fat-free mass, BMI, or z-BMI. Conclusions: Provider-led intervention in child care centers increased preschoolers' PA and reduced adiposity, therefore may represent a viable approach to promoting PA and related health benefits in preschool-aged children. (author abstract)