text: Aloe-emodin, one of the anthraquinones, has been shown to have anticancer activity in different kinds of human cancer cell lines. Therefore, the purpose of this study was to investigate the anti-cancer effect of aloe-emodin on human tongue squamous carcinoma SCC-4 cells. The results indicated that aloe-emodin induced cell death through S-phase arrest and apoptosis in a dose- and time-dependent manner. Treatment with 30 microM of aloe-emodin led to S-phase arrest through promoted p53, p21 and p27, but inhibited cyclin A, E, thymidylate synthase and Cdc25A levels. Aloe-emodin promoted the release of apoptosis-inducing factor (AIF), endonuclease G (Endo G), pro-caspase-9 and cytochrome c from the mitochondria via a loss of the mitochondrial membrane potential (DeltaPsi(m)) which was associated with a increase in the ratio of B-cell lymphoma 2-associated X protein (Bax)/B cell lymphoma/leukemia-2 (Bcl-2) and activation of caspase-9 and -3. The free radical scavenger N-acetylcysteine (NAC) and caspase inhibitors markedly blocked aloe-emodin-induced apoptosis. Aloe-emodin thus induced apoptosis in the SCC-4 cells through the Fas/death-receptor, mitochondria and caspase cascade. Aloe-emodin could be a novel chemotherapeutic drug candidate for the treatment of human tongue squamous cancer in the future.