OBJECTIVES: To assess the impact of a culturally proficient dietary/physical activity intervention on changes in body mass index (BMI) (kg/m 2 ). STUDY DESIGN: Randomized controlled trial (Hip-Hop to Health Jr.) conducted between September 1999 and June 2002 in 12 Head Start preschool programs in Chicago, Illinois. RESULTS: Intervention children had significantly smaller increases in BMI compared with control children at 1-year follow-up, 0.06 vs 0.59 kg/m 2 ; difference -0.53 kg/m 2 (95% CI -0.91 to -0.14), P = .01; and at 2-year follow-up, 0.54 vs 1.08 kg/m 2 ; difference -0.54 kg/m 2 (95% CI -0.98 to -0.10), P = .02, with adjustment for baseline age and BMI. The only significant difference between intervention and control children in food intake/physical activity was the Year 1 difference in percent of calories from saturated fat, 11.6% vs 12.8% ( P = .002). CONCLUSIONS: Hip-Hop to Health Jr. was effective in reducing subsequent increases in BMI in preschool children. This represents a promising approach to prevention of overweight among minority children in the preschool years.