The issue of the role of 2-hydroxyestrone (2-OHE1) in breast cancer has been the subject of considerable controversy as to whether it is carcinogenic or anticarcinogenic. The expanding data base outlined below is most consistent with the conclusion that 2-OHE1 is anticarcinogenic. In every experimental model in which 2-hydroxylation was increased, protection against tumors was achieved. Correspondingly, when 2-hydroxylation was decreased, an increase in cancer risk was observed. Even more dramatically, in the case of laryngeal papillomas induction of 2-hydroxylation with indole-3-carbinol (I3C) has resulted in inhibition of tumor growth during the time that the patients continue to take I3C or vegetables rich in this compound.