Background: Acetabular dysplasia is an important pre-disposing factor for osteoarthritis of the hip. However, it is not completely known how acetabular dysplasia develops during childhood. Objective: To study the prevalence of acetabular dysplasia and its association with body mass index (BMI) and physical activity in 9 year old children. Design: The population for this cross-sectional study was drawn from the ongoing prospective cohort study: Generation R. 9,778 mothers with a delivery date from March 2002 until January 2006 were enrolled. In a random subgroup of these children Dual-energy X-ray absorptiometry (DXA) scanning was performed at age 9. Exposures: BMI, standardized for the Dutch population and categorized in four groups based on extended international Obesity Task Force cut-offs: underweight, normal, overweight and obesity. Physical activity was based on time spent on playing outdoors, playing sports and walking/cycling to school. Main outcomes and measures: The degree of acetabular dysplasia was determined with the centre-edge angle (CEA) and acetabular depth-width ratio (ADR) in DXA images of the hip. Results: 1,188 DXA images of children's hips were available for analysis. The median age of the children was 9.86 years. Prevalence of dysplasia and mild dysplasia was respectively 6.3%; 25.6% with CEA and 4.8%; 25.0% with ADR. BMI was negatively associated with mild dysplasia (OR 0.80 CI 0.71–0.90). Obese children showed less mild dysplasia compared to normal children (OR 0.48 CI 0.24–0.97) in unadjusted analysis. Physical activity represented by walking to school showed a statistically significant negative association with mild dysplasia (OR 0.87 CI 0.76–0.99). After adjustment for age, ethnicity, sex, first born, breech presentation, birthweight, gestational age and Caesarean section, the patterns of association with dysplasia remained for both BMI and physical activity. Conclusions: In this study, being overweight and light physical activity were negatively associated with the development of (mild) acetabular dysplasia at the age of 9 years.