Uterine leiomyomas (uterine fibroids) are sex-steroid dependent benign tumors. Limited knowledge is available regarding the role of estrogen and their receptors in the regulation of fibroids in premenopausal women, and in their shrinkage after treatment with a gonadotropin-releasing hormone analogue (GnRHa). The expression of the two subtypes of the estrogen receptor (ER), ER alpha and ER beta, was studied in leiomyoma and homologous myometrium from women in the proliferative phase of the menstrual cycle and from women treated with GnRHa. The mRNA levels of ER alpha and ER beta were monitored by solution hybridization and in situ hybridization, and receptor proteins were detected and localized by immunohistochemistry. Both ER alpha and ER beta were present in the leiomyomas and homologous myometrium. The ER alpha mRNA level in the leiomyomas was higher than in the surrounding myometrium. The ER beta mRNA level was lower than that of ER alpha in both groups. ER beta immunoreactivity was lower in leiomyomas when compared with the myometrium after GnRHa treatment, while ER alpha was higher in the leiomyomas. The present results imply that the increased ratio of ER alpha/ER beta observed in the fibroids after GnRHa treatment could reflect or be the cause of the shrinkage of the leiomyoma, which is the clinical outcome of this treatment.