Since the publications of Martin, et al. (1948) and Schiff (1959), who were the first to report on the administration of sex hormones to juvenile nasopharyngeal fibroma (JNF) patients, several authors have described the different clinical effects and histologic changes after androgen and estrogen application. Since the mechanism of action of sex steroids in juvenile nasopharyngeal fibroma is almost unknown, the authors have studied androgen receptor binding in cultured tumor fibroblasts from three patients with JNF. Maximum androgen binding (Bmax) of the tumor fibroblasts approximated to that of genital skin fibroblasts, which served as a control androgen target tissue with high receptor density. Furthermore, in vitro experiments showed that the growth rate of tumor fibroblasts increased when testosterone was added to the culture medium, while the addition of two antiandrogens, cyproterone and flutamide, caused a reduction in growth rate. It is concluded from these results that JNF is a hormone-dependent tumor stimulated by testosterone whose growth rate may, at least in vitro, be reduced by antiandrogens such as cyproterone and flutamide.