OBJECTIVE There are still uncertainties about manifestations of early adolescent eating disorders (ED) and their effects. We aimed to determine the prevalence of ED symptoms in early adolescence, derive symptoms dimensions, and determine their effects on social and psychological outcomes and subsequent body mass index (BMI). METHODS Data on 7,082 adolescents aged 13 years from the Avon Longitudinal Study of Parents and Children were obtained on ED symptoms, resulting impairment and family burden and emotional and behavioral disorders using the parental version of the Developmental and Well-being Assessment. Exploratory structural equation models were used to derive ED symptoms dimensions separately by sex and to relate these to contemporary outcomes (impairment, burden, and emotional and behavioral disorders) and a distal outcome (objective BMI at age 15 years). RESULTS Extreme levels of fear of weight gain, avoidance of fattening foods, and distress about weight and shape were common among girls (11%). Three ED symptoms dimensions were identified: bingeing/overeating, weight/shape concern and weight-control behaviors, and food restriction. Bingeing/overeating was strongly associated with higher functional impairment, family burden, and comorbid psychopathology. Bingeing/overeating and weight/shape concern and weight-control behaviors predicted higher BMI 2 years later, whereas food restriction predicted lower BMI. These effects did not change when BMI at age 13 years was included in the model. CONCLUSIONS Eating disorder cognitions are common among young teenage girls. Eating disorder symptoms have adverse cross-sectional and distal consequences, in particular on increasing body weight 2 years later. These findings have important implications for early identification of adolescents engaging in ED behaviors and for obesity prevention.