The estrogens in general use at present are estrone, estriol and estradiol, all of which are prepared primarily from pregnant mare's urine. These preparations are undeniably potent and clinically effective but have certain practical disadvantages: (a) They are expensive. (b) they are relatively ineffective orally and (c) they are difficult to prepare and, except when prepared in pure crystalline form, must be biologically standardized. A cheaper, orally effective, synthetic preparation which could be administered by weight is greatly to be desired. The synthesis of a new estrogen which seemed to meet these criteria was announced by Dodds and his co-workers1 in 1938. It was named diethylstilbestrol because it was derived from stilbene, contained two ethyl groups and was estrogenic. It does not contain the phenanthrene ring nucleus previously thought necessary for estrogenic activity. Preliminary animal studies revealed not only that it was approximately two and one half times as