Background: Effective therapies for the subset of follicular thyroid cancer (FTC) patients with aggressive, metastatic disease are lacking. Therefore, we sought to determine the effects of proteosome inhibition, an emerging class of chemotherapeutic agents, on metastatic FTC cells. Materials and Methods: Human metastatic FTC cells (FTC236) were treated in vitro with the proteosome inhibitor MG132 (0 to 800 nM). Western blot analysis was performed on whole cell lysates isolated after 2 d. To measure cell growth, we performed an MTT cellular proliferation assay over 6 d. Results: Treatment of FTC236 cells with MG132 led to dose-dependent cell growth inhibition. Increases in inactive, phosphorylated GSK-3β, and active β-catenin also were observed. With 800 nM MG132, growth was reduced by 87% at 6 d (P < 0.0001). This reduction in cellular proliferation correlated with the degree of GSK-3β inhibition. MG132 treatment also caused increased p21Waf1/Cip1 and decreased cyclin D1 expression, suggesting that growth suppression may occur through cell cycle arrest. Conclusion: Growth of metastatic human FTC cells appears to be suppressed by proteosome inhibition. Whether this effect is directly due to cell cycle arrest and inactivation of GSK-3β signaling is unclear. Nonetheless, these compounds may become novel treatments for aggressive, metastatic FTC. © 2009 Elsevier Inc. All rights reserved.