The proliferation of MCF-7, human breast cancer cell line, was stimulated by testosterone and estradiol. The aromatase activity in MCF-7 cells, which catalysed the conversion of testosterone to estradiol, was inhibited by a novel non-steroidal aromatase inhibitor, YM5111, with the IC50 of 0.2 nM, indicating that its inhibitory activity was 5.5 times more potent than that of CGS 16949A. YM511 inhibited the proliferation of MCF-7 stimulated by testosterone but did not inhibit the cell proliferation stimulated by estradiol. The IC50 values of YM511 for cell growth and DNA synthesis were 0.13 nM and 0.18 nM, respectively, demonstrating that YM511 was about 3-5 times more potent than CGS 16949A and had no anti-estrogenic or cytotoxic activity. YM511 significantly inhibited testosterone-stimulated transcriptional activation of estrogen-responsive element (ERE) in MCF-7 cells transfected transiently with ERE-luciferase reporter plasmid. The IC50 of YM511 for transactivation was 0.36 nM, suggesting that its inhibitory potency was comparable to the inhibition of aromatase activity of MCF-7 cells. These data may indicate that the inhibition by YM511 of cell proliferation of MCF-7 is attributed to the decreased production of estrogen due to the inhibition of aromatase activity. YM511 may be useful in the treatment of estrogen-dependent cancers.