The interstitial cells of the testicle, which were first described by Leydig1 in 1850, may rarely give rise to tumors, some of which are accompanied with endocrine changes. These cells have been regarded generally as the primary source of androgen. The study of such tumors is of interest, therefore, and may contribute to the knowledge of the normal endocrine physiology of the testis. This paper is concerned primarily with a review of the hormonal manifestations described in the previously reported cases of interstitial cell tumor and a report of 3 new cases, in which the patients were adults, one of whom had gynecomastia. In addition, we have elaborated certain criteria for the recognition of these tumors. LITERATURE Several comprehensive reviews of the literature of interstitial cell neoplasia have been published recently, notably by Jemerin,2 Fialho3 and Warren and Olshausen.4 However, these writers and others have disagreed