The nuts of Semicarpus anacardium (Anacardiaceae) are one of the most favoured medicine in the Indian System of Medicine for the management of arthritis and several other free radical mediated diseases. It is also recommended for the management of the breast cancer. In this report we have investigated its role on the cell cycle and cell viability on the DU-145 cells (transformed prostate cells) by flow cytometric technique. It was observed that the plant extract significantly arrests the cell cycle at G-1 stage, and induced apoptosis. On higher concentrations, it affects the cell viability. The response was dose dependent.