Serum levels of testosterone, estradiol, progesterone, 17α-hydroxyprogesterone, follicle-stimulating hormone, and luteinizing hormone were measured in 16 boys with pubertal gynecomastia. Six patients had elevated serum estradiol concentrations, and four of these six also had elevated progesterone levels. Serum estradiol/testosterone ratios were high for the stage of puberty in 11 of the 16 patients. In five patients who had two or more determinations, the steroid concentrations returned toward or into the normal range. Transient increases in serum estradiol concentration or an abnormally high estradiol/testosterone ratio may be causally related to the development of gynecomastia in adolescent boys. © 1975 American Medical Association. All rights reserved.