This report represents the first study in the literature linking development of severe gynecomastia, in a 17 1/2-yr-old boy, to high levels of aromatase expression in a large fibrolamellar hepatocellular carcinoma, which gave rise to extremely elevated serum levels of estrone (1200 pg/mL) and estradiol-17 beta (312 pg/mL) that suppressed FSH and LH (1.3 and 2.8 IU/L, respectively), and consequently testosterone (1.53 ng/mL).