The concentrations of 17 beta-estradiol, progesterone, and estrone in leiomyomas and tumor-bearing myometrium were determined by specific radioimmunoassays in hysterectomy specimens from 12 women aged between 30 and 47 years. The concentration of 17 beta-estradiol was significantly higher in leiomyomas than in myometrium. There was no difference in the concentration of progesterone in leiomyomas and myometrium. The concentration of estrone was higher in the myometrium than in the leiomyomas, although the difference was not statistically significant. The concentrations of progesterone, 17 beta-estradiol, and to some extent estrone show variation with the phase of the menstrual cycle. The significance of the difference in concentration of 16 beta-estradiol in leiomyomas and myometrium, the relationship of this difference to the concentrations of estrogen and progesterone receptors, and factors possibly related to the growth of leiomyomas are discussed.