Abstract Background. Social desirability (SocD) may bias children's self-reported health behaviors and attitudes and confound relationships with health outcome measures. Methods. Ninety-five, 8- to 10-year-old African-American girls completed dietary recalls, a physical activity checklist, psychosocial questionnaires related to diet, and physical activity; and 3 days of physical activity monitoring. Potential SocD construct bias was investigated by comparing designated criterion measures of physical activity, beverage intake, and body mass index (BMI) with respective self-reported measures related to activity, beverage preferences, and body image and weight concerns in cross-sectional regression models. Potential confounding by SocD of associations between self-reported behaviors with BMI was assessed using change-in-coefficient regression analyses. Results. Controlling for age and BMI, overestimates of self-reported activity ( P = 0.02), underestimates of sweetened beverage preferences ( P = 0.02), and lower ratings of weight concerns and dieting behaviors ( P's < 0.05) were related to SocD. Confounding by SocD of associations between self-reported physical activity and energy intake with BMI was found. Conclusions. In 8- to 10-year-old African-American girls, SocD was found to bias self-reports of diet and physical activity and confound associations between BMI and self-reported physical activity and energy intake. Methods to measure and control SocD bias are needed to reduce potential distortion of relationships between diet and physical activity and health outcomes.