"Dieting is common among adolescent girls and may place them at risk of using unhealthy weight-control behaviors (UWCBs), such as self-induced vomiting, laxatives, diet pills, or fasting. Research has suggested that social factors, including friends and broader cultural norms, may be associated with UWCBs. The present study examines the relationship between the school-wide prevalence of current weight loss efforts among adolescent girls, friends' dieting behavior, and UWCBs, and investigates differences in these associations across weight categories. Survey data were collected in 31 middle and high schools in ethnically and socio-economically diverse communities in Minnesota, USA. The response rate was 81.5%. Rates of UWCBs were compared across the spectrum of prevalence of trying to lose weight and friends' involvement with dieting, using chi(2) analysis and multivariate logistic regression, controlling for demographic factors and clustering by school. Girls with higher body mass index (BMI) were more likely to engage in UWCBs than those of lower BMI. Multivariate models indicated that friends' dieting behavior was significantly associated with UWCBs for average weight girls (OR = 1.57, CI = 1.40-1.77) and moderately overweight girls (OR = 1.47, CI = 1.19-1.82). The school-wide prevalence of trying to lose weight was significantly, albeit modestly, related to UWCBs for average weight girls (15th-85th percentile; OR = 1.17, CI = 1.01-1.36), and marginally associated for modestly overweight girls (85th-95th percentile; OR = 1.21, CI = .97-1.50), even after controlling for friends' dieting behaviors. The social influences examined here were not associated with UWCBs among underweight ( < 15th percentile) or overweight ( > 95th percentile) girls. Findings suggest that social norms, particularly from within one's peer group, but also at the larger school level may influence UWCBs, particularly for average weight girls. Implications for school-based interventions to reduce UWCBs are discussed."