The two estrogen receptors, ERalpha and ERbeta, are likely to have roles in the pathophysiology of fibroid development. They have been detected in myometrial and leiomyoma (fibroid) tissue, but the cell types expressing ERalpha and ERbeta have not been determined. ERs have also been detected in human endothelial cells. The aims of the present study were to determine whether pure populations of myometrial microvascular endothelial cells (MEC) express ERalpha and ERbeta, to compare MEC ERalpha/ERbeta expression with that of pure populations of myometrial smooth muscle cells (SMC) and to determine if ERalpha/ERbeta are differentially expressed in MEC and SMC of myometrium and fibroids from nine paired samples. Using RT-PCR (for ERalpha and ERbeta) and Western blotting (for ERalpha only), we demonstrated that all cultures of early passage myometrial and fibroid SMC (>99% pure) expressed ERalpha but not ERbeta, while myometrial and fibroid MEC (>99% CD31+) constitutively expressed ERbeta. However, both myometrial and fibroid MEC showed variable expression of ERalpha, with approximately 60% of MEC samples expressing ERalpha. While the majority (6/9) of MEC from myometrial and fibroid pairs demonstrated the same pattern of ERalpha expression, 3/9 pairs showed discordant ERalpha expression. These results show that ERalpha and ERbeta are differentially expressed in SMC and MEC of human myometrium and fibroids. Since ERalpha and ERbeta mediate opposing transcriptional activities, any effect of estrogen on the growth and development of fibroids is likely to be complex and may involve both SMC and MEC.