Inaccurate reporting of energy intake makes it difficult to study the associations between diet and weight status. This study examined reported energy intake at age 9 years as a predictor of girls' body mass index (BMI) at age 11 years, before and after adjusting for parents' BMI and girls' pubertal status. This prospective, observational cohort study included 177 non-Hispanic white girls and their parents. When the subjects were 9 years of age, three 24-hour recalls were used to categorize girls as plausible or implausible over-reporters and under-reporters based on previously published methods. Height and weight was measured to calculate BMI. Linear and hierarchical regression analyses were used to predict girls' BMI. Results revealed that girls who under-reported had significantly higher BMIs than plausible and overreporters. Among the total sample and among implausible reporters, reported energy intake was not a significant predictor of BMI; however, among plausible reporters, reported energy intake explained 14% of the variance in BMI and remained a significant predictor after adjusting for parental BMI and girls' pubertal status. Systematic bias related to underreporting in dietary data can obscure relationships with weight status, even among young girls. A relatively simple analytical procedure can be used to identify the magnitude and nature of reporting bias in dietary data.