Few studies have been carried out on emerging adults’ friendships and on their developmental roots. Research suggests that in adolescence, both attachment to parents and attachment to peers play a role in future socio-emotional development. The aim of the present study was to compare attachment in these two types of relationships in adolescence according to gender and test whether they respectively predicted the perception of best friendship in early adulthood. A sample of 83 participants (49 girls) was seen in early adolescence (M = 13.66 years, SD = 0.64) and 7 years later (mean age = 21.15 years, SD = 0.83). At T1, participants completed the Inventory of Parent and Peer Attachment, which measures attachment using three subscales (communication, alienation and trust) and one global security score. At T2, they completed the McGill Friendship Questionnaire. Results show that in adolescence, boys report higher security with parents compared to peers (mainly due to better communication), unlike girls who obtain higher scores with peers. Longitudinal findings reveal that alienation in the relation with parents is what best predicts friendship quality in early adulthood. These findings underline the specific internal working models at play in socio-emotional development and the way gender differences evolve from adolescence to early adulthood.