The aim of the current study was first to investigate cytotoxic activity of usnic acid (UA) on hormone-dependent breast and prostate cancer, and normal cells. Cells were treated with increasing concentrations (25 to 150 mu M) of UA for 48hours and cell viability, quantitative and morphological analysis of cell death, and cell cycle analysis were performed. UA was shown to have selective cytotoxicity on hormone-dependent cancer cells with the IC50 levels of 71.4 and 77.5 mu M for MCF7 and LNCaP cells, respectively. UA induced apoptotic cell death and G0/G1 cell cycle arrest without damaging normal cells. MCF7 cells were more sensitive to UA than LNCaP cells. Our results first revealed that UA is a promising candidate as an alternative agent for hormone-dependent breast and prostate cancers. However, molecular mechanism underlying the UA-mediated cell death in cancer cells should be investigated further.