An interstitial-cell tumor of the testis with estrogen activity is reported. Evidence for estrogen secretion was based on clinical grounds as well as on a study of steroid excretion. The function of both the tubular and the endocrine apparatus of the testis was apparently suppressed as a result of the production of estrogen by the tumor. There was a return to normal function after removal of the tumor. The literature on the subject is reviewed and discussed.