Anaplastic thyroid cancer (ATC) is an extremely aggressive tumor characterized by marked epithelial mesenchymal transition, which leads, almost invariably, to death. Peroxisomal proliferator-activated receptor (PPAR)-gamma agonists have recently emerged as potential antineoplastic drugs. To establish whether ATC could be a target of PPAR gamma agonists, we first examined PPAR gamma protein expression in a panel of six ATC cell lines and then studied the biologic effects of two PPAR gamma agonists, ciglitazone and rosiglitazone, that belong to the class of thiazolidonediones. PPAR gamma protein was present and functional in all ATC cell lines. Both ciglitazone and rosiglitazone showed complex biological effects in ATC cells, including inhibition of anchorage-dependent and -independent growth and migration, and increased apoptosis rate. Rosiglitazone-induced growth inhibition was associated with cell cycle arrest and changes in cell cycle regulators, such as an increase of cyclin-dependent kinases inhibitors p21(cip1) and p27(kip1), a decrease of cyclin D1, and inactivation of Rb protein. Rosiglitazone-induced apoptosis was associated with a decrease of Bcl-X(L) expression and caspase-3 and -7 activation. Moreover, rosiglitazone antagonized IGF-I biological effects by up-regulating phosphatase and tensin homolog deleted from chromosome 10 with subsequent inhibition of the phosphatidylinositol 3-kinase/Akt signaling pathway. Finally, rosiglitazone increased the expression of thyroid-specific differentiation markers. In conclusions, these data suggest that PPAR gamma agonists induce a partial reversion of the epithelial mesenchymal transition in ATC cells by multiple mechanisms. PPAR gamma agonists may, therefore, have a role in the multimodal therapy currently used to slow down ATC growth and dissemination.