Background—Little is known about the actual physical activity and screen time behaviors of an adolescent’s friends relative to the individual’s behavior. Purpose—To determine the associations between an adolescent’s physical activity and screen time and his/her nominated friends’ physical activity and screen time. Methods—Data were obtained from EAT 2010 (Eating and Activity Among Teens), a large cross-sectional study (n=2126) conducted in 20 middle schools and high schools in Minneapolis/ St. Paul MN during the 2009–2010 academic year and analyzed during 2011. Each participant nominated up to six friends from a school roster, and data from those friends were obtained as part of the school-based data collection procedures. Physical activity and screen time were assessed with previously used and validated questionnaires. Generalized estimating equation models, stratified by gender, were used to assess associations between adolescents’ physical activity and screen time and their friends’ physical activity and screen time. Results—Physical activity for female adolescents was associated with their male and female friends’ physical activity, including their male and female best friends (all p<0.05). Males’ physical activity was associated with their female friends’ physical activity (p<0.03). Females’ screen time was associated with their male and female friends’ screen time (p≤0.03), but not with that of their best friends. Males’ screen time was associated with only their female friends’ screen time (p=0.04). Conclusions—The consistent association between female adolescents’ physical activity and their friends’ physical activity indicates a need to include peer effects on adolescent female physical activity in future intervention work.