Statins, hydroxy-methyl-glutaryl coenzyme A reductase inhibitors, are the most effective medication for lowering cholesterol, cardiovascular morbidity and mortality. On the basis of our previous in vitro experimental results on an anaplastic thyroid cancer cell line, we designed a nude mouse model in which cancer cells were seeded subcutaneously to examine the potential anticancer effects of lovastatin in vivo. As expected, tumor growth was significantly reduced in the mice treated with 5 or 10 mg/kg/day of lovastatin compared with the positive control group. However, the tumor grew much faster in the mice treated with 1 mg/kg/day of lovastatin than in the positive control group. We suspect this result might be related to vascular endothelial growth factor. In this model, we found that lovastatin inhibits tumor growth at a high dosage (5 or 10 mg/kg/day), suggesting it could be used as an effectively adjuvant chemotherapy for cancer. However, it also promotes tumor growth at a low dosage (1 mg/kg/day). This duality effect should be further studied for patients treated with various dosages of statins.