Flaxseeds have been shown to play a role in the prevention of cancer and heart disease, and it is believed that their more favorable fatty acid composition is responsible. Sprouting is a natural method to modify nutritional components and to decrease cyanide poisoning of raw flaxseeds. Here, we investigated the in vitro effects of flaxseed sprouts on cell growth and apoptosis of human breast cancer cells. In a series of in vitro experiments, estrogen-receptor-positive (MCF-7) and estrogen-receptor-negative (MDA-MB-231) cells were cultured and treated with flaxseed sprouts, and then cell proliferation, apoptosis, and gene expression were measured. Flaxseed sprouts significantly reduced the growth of both of MCF-7 and MDA-MB-231 cells and also increased apoptosis. However, flaxseed sprouts did not affect the growth of MCF-10A mammary epithelial cells. In gene transcription analysis using quantitative real-time polymerase chain reaction, flaxseed sprout treatment significantly upregulated p53 mRNA in both cell cancer lines. These results suggest that flaxseed sprouts induce apoptosis and inhibit cancer cell growth, thereby demonstrating their anti-proliferative effects in breast cancer cells. This study may provide important information for devising dietary strategies to reduce breast cancer risk.