Resveratrol is a naturally occurring product found in grapes and wine. The effect of synthetic resveratrol on the growth of estrogen receptor (ER)-positive (KPL-1 and MCF-7) and -negative (MKL-F) human breast cancer cell lines was examined. Resveratrol at low concentrations caused cell proliferation in ER-positive lines (KPL-1, ≤22 μM; MCF-7, ≤4 μM) whereas at high concentrations (≥44 μM) it caused suppression of cell growth in all three cell lines examined. Growth suppression was due to apoptosis as seen by the appearance of a sub-G1 fraction. The apoptosis cascade up-regulated Bax and Bak protein, down-regulated Bcl-x_L protein, and activated caspase-3. Resveratrol (52–74 μM) antagonized the effect of linoleic acid, a potent breast cancer cell stimulator, and suppressed the growth of both ER-positive and -negative cell lines. Thus, resveratrol could be a promising anticancer agent for both hormone-dependent and hormone-independent breast cancers, and may mitigate the growth stimulatory effect of linoleic acid in the Western-style diet.