To explain the sensitivity of uterine leiomyomata to endogenous and exogenous estrogens, specific cytosol receptor proteins for estradiol-17β and progesterone were found in tissue from uterine leiomyomata and compared with the concentrations of receptors in normal myometrium and endometrium from the same patient. The concentration of cytoplasmic estrogen receptors in leiomyomata (40.5 ± 7.6 fmole/mg cytosol proteins, mean ± SEM) was significantly greater than that found in myometrium (16.5 ± 1.5) and significantly less than that found in endometrium (131 ± 15.1). The concentration in leiomyomata of cytoplasmic progesterone receptors (104.4 ± 41.2 fmole/mg cytosol protein, mean ± SEM) was also greater than that found in myometrium (80.0 ± 29.9) and less than that of endometrium (194.7 ± 5.5); however, individual variation precluded statistical significance. The presence of cytoplasmic receptor proteins in myomatous tissues explains the sensitivity of the leiomyoma to endogenous or exogenous steroids and may provide information useful in the clinical management of this neoplastic disorder.