This study aimed to determine the prevalence rate of gynecomastia, determine mean glandular breast tissue sizes, and evaluate whether there is any difference in the prevalence rate of gynecomastia according to age using three different reference values of glandular breast tissue size (≥5, ≥10, ≥20 mm) in the pediatric age group. Glandular breast tissue sizes were measured retrospectively from thoracic computed tomography (CT) images taken for other reasons in 961 boys aged 1-18 years. When each breast was evaluated separately (1,922 breasts), gynecomastia was observed in 1,001 (52.1%), 719 (37.4%), and 216 (11.2%) breasts with ≥5, ≥10, and ≥20 mm considered as reference values, respectively. A significant difference was found in terms of gynecomastia (p<0.001) and mean glandular breast tissue size (p<0.001) with respect to age. New studies are currently needed to determine the glandular breast tissue size and the prevalence rate of gynecomastia in boys, and thoracic CT images can be used for this purpose.