Objective: To assess the association between weight concerns and weight control practices of adolescents and their mothers. Design and Participants: Cross-sectional study of 5331 adolescent girls and 3881 adolescent boys (age range, 11.8-18.4 years) in an ongoing cohort study and their mothers. Participants were included in the analysis if both the adolescent and his or her mother returned a questionnaire mailed in 1999 and provided information on weight, height, and weight concerns. Results: More adolescent girls (33.0%) than boys (8.1%) thought frequently about wanting to be thinner. Compared with adolescent girls who accurately perceived that their thinness was not important to their mother, girls who misperceived (odds ratio [OR]=1.9; 95% confidence interval [CI], 1.3-2.8) or accurately perceived (OR=2.85; 95% CI, 1.0-8.4) that it was important to their mother that they be thin were significantly more likely to think frequently about wanting to be thinner. Among the adolescent boys, only those who accurately perceived that it was important to their mother that they not be fat were more likely than their peers to think frequently about wanting to be thinner (OR=3.8; 95% CI, 2.3-6.2). Adolescents who accurately perceived that it was important to their mother to be thin or not fat were significantly more likely to be frequent dieters than their peers who accurately perceived that their weight was not important to their mother. Conclusion: Among adolescents, an accurate perception that weight status is important to their mother is associated with thinking frequently about wanting to be thinner and about frequent dieting.