Objective: Attention-deficit hyperactivity disorder (ADHD) is one of the most common psychiatric disorders that begins in childhood. By using self-esteem and loneliness scales, our aim was to compare the adolescents who were diagnosed early and are currently receiving treatment with the ones with late diagnoses. By examining loneliness and self-esteem directly, we wish to emphasize a possibility of positive impact of early diagnosis and treatment. We hypothesized that the duration of treatment is an important factor in reducing several negative psychosocial impacts of ADHD. Methods: The study included 62 adolescent patients with a late ADHD diagnosis between the ages of 12 and 14 years, and 55 adolescent patients with early diagnosis between the ages of 6 and 8 years who have been followed up regularly with appropriate treatment for nearly 6 years. These groups were compared with regard to self-esteem and feelings of loneliness. Results: The UCLA Loneliness Scale scores were higher, but the Rosenberg Self-esteem Scale scores were lower in the late-diagnosed group. These results did not change with regard to ADHD subtypes and gender. Conclusion: The late-diagnosed adolescents with ADHD feel they are alone more than the early-diagnosed adolescents, and they like themselves less compared to the early-diagnosed group. [Anadolu Psikiyatri Derg 2017; 18(1.000): 79-84]