Cultivated “Winter-worm and summer-grass (WWSG)” is one of the most valued traditional Chinese medicines for the replenishment of body health. It consists of the entomopathogenic fungus Cordyceps militaris and its artificial host Bombyx mori. The water extract from cultivated WWSG inhibits human breast cancer MCF-7 cell proliferation in a dose and time dependent manner, with IC₅₀ values at about 18 μg/ml when MCF-7 cells were incubated with C. militaris-containing medium for 96 hrs. Although C. militaris also prevented normal breast cells MCF-10A from proliferation, MCF-10A cells were able to quickly recover to their normal proliferation rate when the cells were removed from C. militaris-containing medium. RT-PCR analysis revealed that C. militaris induced the transcriptional expression of pro-apoptotic marker genes in MCF-7 cells incubated with C. militaris for 24 hrs or longer, resulting in subsequent DNA fragmentation as detected by TUNEL system. These findings suggest that C. militaris inhibits breast cancer cell proliferation through apoptosis by inducing pro-apoptotic marker gene expression.