A 28-year-old man with evidence of feminization was demonstrated after 4 years of investigation to have an estrogen-secreting interstitial cell tumor. Such feminizing neoplasms are uncommon, only 37 having been described. They are usually benign and are characterized by gynecomastia, a testicular mass and, with lesser frequency, by decreased libido and potency and poor semen quality. The urinary excretion and plasma levels of estrogen are increased and, by selective testicular catheterization, the site of increased estrogen production can be localized. Secondary histologic changes occur in the nontumorous portions of the testis as well as in the contralateral testis; they are most marked in the area immediately adjacent to the tumor. Postoperatively, the gynecomastia regresses, the excessive levels of estrogen return to normal, libido improves, and the sperm count increases to normal.

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