Cioteronel [CPC 10997, Cyoctol®, X-Andron] is an antiandrogen agent which was in phase II trials for androgenetic alopecia (male pattern baldness), and acne. It was also under development for the oral treatment of benign prostatic hyperplasia, but it was discontinued due to poor efficacy. CPC 10997 was found to be effective in vitro as an antiandrogen without effects on either the estrogen or the progesterone receptors in carcinomas of the breast, ovary and prostate as well as in malignant melanomas. CPC 10997 was found to be more effective against carcinomas of the breast, the kidney, the ovary and the prostate than conventional antineoplastic agents in the majority of tumors tested. The antineoplastic action of CPC 10997 appears to be the inhibition of RNA synthesis, but it has no detectable untoward effects on nonneoplastic cells, in vitro. Although it blocks competitively the binding of dihydrotestosterone (DHT) to its protein receptor, it has no significant effects on either estrogen or progesterone receptors. In view of the very favorable toxicology profiles and in vitro efficacy, further trials using CPC 10997 as an antineoplastic agent are indicated.