Purpose: Follow-up studies of children diagnosed with autism spectrum disorder (ASD) in early childhood that focus on friendship formation during adolescence are scarce. The present study focused on exploring characteristics possibly related to the ability to establish friendships during adolescence among children diagnosed with ASD in toddlerhood. Methods: The cohort included 43 participants who underwent comprehensive assessments during toddlerhood and adolescence. Participants were divided into two groups [Friendship(+)/Friendship(-)] based on (1) adolescent social insight as assessed by professionals and (2) parental and adolescent self-reports regarding having or not having friends. No differences in IQ, ASD symptoms, or adaptive behavior during early childhood were found between the two groups. Results: Different and better changes in social communication, adaptive socialization, and daily living skills were observed for the Friendship(+) group. Adolescents with ASD in the Friendship(+) group exhibited greater social independence. Attention-deficit/hyperactivity disorder incidence, anxiety symptom severity, and placement in mainstream or special education classes did not differ between the two groups. Conclusion: This long-term study highlights that for children with ASD, longitudinal growth in social communication and adaptive functioning is possible, highly important for and related to the development of the complex ability to establish friendship.