Apoptosis is a physiological mechanism for eliminating malignant cells, including cancer cells, without eliciting damage to normal cells or surrounding tissues. Here, we report that rhein (4,5-dihydroxyanthraquinone-2-carboxylic acid), a major constituent in the rhizome of rhubarb, induced apoptosis of human nasopharyngeal carcinoma (NPC) cells. Rhein induced apoptosis in NPC cells as demonstrated by increased nuclear condensation and DNA fragmentation. Moreover, for the first time in NPC cells it was demonstrated that the pathway involved in rhein-induced apoptosis is caspase-dependent, presumably through the endoplasmic reticulum (ER) stress pathway, as shown by an increase in the levels of glucose-regulated protein 78 (GRP 78), PKR-like ER kinase (PERK), activating transcription factor 6 (A TF6) and CCAA TIenhancer-binding protein homologous protein (CHOP) as well as the activation of caspase-3, -8, -9 and -12. This increased susceptibility to ER stress-induced apoptosis may be due to an increased accumulation of reactive oxygen species (ROS). Rapid accumulation of calcium (Ca2+) and a decrease in the mitochondrial membrane potential (MMP) were also observed. Cytochrome c and apoptosis-inducing factor (AIF) were released upon treatment with rhein. Taken together, these results suggest that ER stress and Ca2+-dependent mitochondrial death pathway may be involved in rhein-induced apoptosis in NPC cells.