Supportive friendships are an invaluable resource for adolescents because of their various developmental tasks, but establishing and maintaining them requires particular interpersonal skills. In order to identify social-emotional competencies associated with having and losing friends, N = 206 early adolescents (12-14 years of age) were examined longitudinally right after the transition to secondary school in the beginning of grade 7 and again at the end of this school year. Adolescents who had at least one reciprocal friend at both times were compared to those who lost all their friends over the school year. Repeated-measures MANOVAs indicate effects of time (and development) that underline an increase of self-disclosure, explanations and humor as well as a general decrease of openly aggressive behavior when angry at the friend. Interaction effects suggest that adolescents who lost their friend(s) intensified their use of physically and relationally aggressive behavior and of fantasies of revenge over time while continuously befriended adolescents decreased in their aggressive behavior when coping with anger in their friendships. Adolescents who had friends at both times were more willing to disclose private information whereas the self-disclosure of adolescents who lost their friends stagnated. These results are discussed in terms of deficient social-emotional competencies as the cause and the consequence of losing friends.