Pubertal gynecomastia occurred in 20 to 29 boys who were followed for 24 or more months during puberty. Mean concentrations of LH, FSH, prolactin, testosterone, and estrone and estradiol were compared. Although levels did not differ between boys who developed gynecomastia and those who did not, when compared with the stage of puberty, changes were noted between mean concentrations from samples before and when gynecomastia was first noted. A significant increase of estradiol occurred with the onset of gynecomastia, while testosterone levels did not change; thus the testosterone-estradiol ratio decreased. Prolactin concentrations fell significantly with the onset of gynecomastia. These data indicate a difference of hormonal dynamics between boys with and without gynecomastia.