Shikonin, isolated from the plant Lithospermum erythrorhizon Sieb. Et Zucc, has been reported to induce apoptosis in several tumor cells. However, such effect of shikonin on human breast cancer cells has not been reported. Thus, in the present study, whether shikonin could induce MCF-7 human breast cancer cell apoptosis was investigated. The results showed that shikonin (2.5—80 μM) induced MCF-7 cell death in a time- and dose-dependent manner, as measured by MTT assay. The IC₅₀ of a 24 h, 48 h and 72 h time course for MCF-7 cells was 7.4±0.4, 6.3±0.6 and 3.9±0.5 μM, respectively. Cellular morphology observation showed that MCF-7 cells underwent marked apoptotic morphological changes upon treatment with 10 μM shikonin compared with the untreated control. Flow cytometric analysis of shikonin-treated MCF-7 cells showed that the ratio of the apoptotic DNA fragmentation increased in a dose-dependent manner. The present study demonstrated for the first time that the cytotoxic effect of shikonin on MCF-7 cells underwent apoptosis process.