Introduction Prolactinoma is the most common functioning pituitary adenoma. However, there have been limited studies for prolactinoma in adolescents. The aim of the study is to assess the characteristics of Korean adolescents with prolactinoma and their clinical course. Methods: We reviewed the medical records of patients who were diagnosed with prolactinoma before 20 years old at Samsung medical center during a 13-year period (2005-2017). Twenty five patients with prolactinoma (median age=16.9, ranged from 10.1 to18.5; 20 female/ 5 male) were divided into two groups according to tumor size (11 patients with microadenoma and 14 patients with macroadenoma), and factors related to tumor size were evaluated. In addition, the characteristics between surgery group (n=14) and non-surgery group (n=11) were compared. Results: The most common clinical manifestations included galactorrhea (32%), amenorrhea (28%), followed by visual disturbance (16%), and headache (12%). Serum prolactin ( PRL) levels at diagnosis ranged from 17.2 to 10,000 ng/mL. Tumor size varied from 4 to 48 mm. After surgical therapy, a mean immediate postoperative decline rate in PRL level was 93.5%. No differences in gender, age at diagnosis, ki-67 index, and nadir PRL level were found between microadenoma and macroadenoma groups. PRL level at diagnosis was significantly higher in macroadenoma group (516 vs 114.2 ng/mL, P 0.001). Male gender, PRL level at diagnosis, and immediate postoperative PRL level were positively correlated with maximal tumor diameter (r=0.536, P=0.006; r=0.729, P0.001; r=0.795, P0.001, respectively). Postoperative decline rate in PRL level was negatively correlated with maximal tumor diameter (r=-0.655, P=0.008). Maximal tumor diameter and PRL level at diagnosis were significantly higher in surgery group in comparison with non-surgery group ( P=0.001, P =0.013, respectively). Conclusions: We found that male gender is at higher risk for macroadenoma more than female in adolescents with prolactinoma. Since PRL levels are not adjusted well after surgery in the macroadenoma group, prolonging medical treatment and close monitoring are essential for remission and early detection of recurrence. Given that diagnosis and prognosis may vary depending on the gender, we need to consider a more aggressive treatment in males.