Adolescents may be influenced both by their friends' behaviors and by the features of their friendships. To examine both types of influence, seventh and eighth graders (N = 297) were asked in the fall of a school year to report their involvement and disruption at school. The students also described the positive and negative features of their best friendships. Teachers reported on the students' involvement, disruption, and grades. These assessments were repeated in the following spring. Students whose friends in the fall described themselves as more disruptive increased in self-reported disruption during the year. Girls' self-reported disruption was more influenced by that of their very best friend than was boys'. Students whose very best friendships had more positive features increased in their self-reported involvement during the year. Students whose friendships had more negative features increased in their self-reported disruption, but only if their friendships also had many positive features. The theoretical and practical implications of these findings, and the adequacy of different methods for estimating friends' influence, were discussed.