Tremella mesenterica (TM) is a common food and folk medicine widely used in several Asian countries as a tonic for the lungs. In the present study, we compared the effects of extracellular polysaccharides (EPS), intracellular polysaccharides (IPS), and ethanol extract (EE) of Tremella mesenterica on the induction of apoptosis into human lung carcinoma A549 epithelial cells. The EE, but not the EPS or the IPS, almost completely inhibited the growth of A549 cells. The results of Annexin V-FITC/PI staining and flow cytometric analysis indicated that the percentage of Annexin V(+)/PI(-) cells in EE-treated cells increased to 32.8%. The results of further investigation showed a disruption of mitochondrial transmembrane potential (DeltaPsi(m)), the production of reactive oxygen species (ROS), and the activation of caspase-3 protein in EE-treated cells. These findings suggest that EE can decrease cell viability and induce apoptosis in A549 cell lines by activating a mitochondrial pathway.