Simvastatin (SIM), a widely used cholesterol-lowering drug, also exhibits tumor-suppressive potentials in several types of malignancy. Colorectal cancer (CRC), the third most common malignant neoplasm, accounts for the second most leading cause of cancer-related deaths worldwide. In the present study, we investigated the anticancer effects of SIM on CRC using primary cancer cells lines (CPs: CP1 to CP5) isolated from five Taiwanese colorectal cancer patients as a model for colorectal cancer. We treated all five CPs with SIM for 24-72hr and observed the respective cell viability by an MTT assay. SIM increased DNA content of the G(1) phase, but did not induce apoptosis/necrosis in CPs as shown by flow cytometry with propidium iodide (PI)/annexin V double staining and PI staining. The expression of G(1) phase-related proteins was analyzed by RT-PCR and Western blotting. SIM suppressed cell growth and induced cell cycle G(1)-arrest by suppressing the expression of CDK4/cyclin D1 and CDK2/cyclin E1, but elevating the expression of glycogen synthase kinase 3 in CPs. Our findings indicate that SIM may have antitumor activity in established colorectal cancer.