Ganoderma lucidum, a mushroom that has been used to treat disease in East Asia for centuries, has been shown to be effective against many types of tumors, but the exact cellular mechanism of action is unknown. In this study we examined proliferation of a lung cancer cell line after treatment with 12 concentrations of powdered G. lucidum for 24, 48, and 120 hours. Based on half-maximal inhibitory concentrations values, proliferation of the H1793 cell line seemed to be sensitive to the extract in a time- and dose-dependent manner. We used immunoblot analysis to examine the amounts of cell cycle proteins (cyclin D, Cdk4, and Cdc2) and apoptotic proteins (Bcl-xL and Bax) after treatment with a range of G. lucidum concentrations. Changes in amounts of proteins that regulate the cell cycle were consistent with longer G1 and G2 phases. Proapoptotic protein (Bax) levels increased 6.5-fold, with a commensurate increase in the Bax-to-Bcl ratio, especially at 48 and 120 hours. These results suggest that the decrease in cellular proliferation correlated with a change in both cell cycle progression and apoptosis, and that the triterpenoid in G. lucidum is the bioactive component. Further biochemical characterization of this ancient herbal remedy could hold promise for treating lung cancer.