This study examined the influence of Thymus vulgaris L. extract on estrogeninduced proliferation of estrogen receptor-positive breast cancer cells by using the MCF-7 human breast adenocarcinoma cell test system. The influence of Thymus vulgaris L. extract on the proliferation of the MCF-7 cells was determined using the alamar blue assay. 17β-estradiol (0.01~100 nM) induced cell proliferation in MCF-7 cells at concentrations of 1, 5 and 10 nM without cytotoxic activity. An ethanol extract of Thymus vulgaris L. slightly increased cell proliferation, but the increase in cell proliferation was not statistically significant. However, the Thymus vulgaris L. extract significantly increased cell proliferation in combination with 17β-estradiol. These findings suggest that Thymus vulgaris L. extracts may provide a safe, natural remedy for menopause by increasing the sensitivity of 17β-estradiol response on estrogen receptor-positive cells.