Background: Anaplastic thyroid cancer (ATC) is a very aggressive thyroid gland malignancy with very poor prognosis. It is suspected that the Notch signaling pathway, which is not active in ATC, may have a tumor suppressor function in this neoplasm. However, it remains unknown whether activation of Notch can yield therapeutic efficacies in ATC. METHODS: The purpose of this study was to evaluate the effect of chrysin, a potential Notch inducer identified via high-throughput screening, on ATC both in vitro and in vivo. RESULTS: Chrysin treatment of ATC cells led to a dose-dependent inhibition of cellular growth. Protein and messenger RNA levels of Notch1 and Hes1 (hairy/enhancer of split 1), a downstream Notch1 effector, were both up-regulated with treatment. Luciferase reporter assays incorporating the C promoter-binding factor 1 (CBF1) binding site also confirmed the functional activity of chrysin-induced Notch1. Oral administration of chrysin suppressed the growth of ATC xenografts by an average of 59% compared with the vehicle control group (P =.002). In addition, calculated median time to tumor progression was 11 days for control mice and 21 days for the chrysin treatment group (P =.008). Analysis of chrysin-treated ATC tumors revealed an increase in the active intracellular domain of Notch1 protein. Activation of Notch1 in vivo was associated with the induction of cleaved Poly ADP ribose polymerase (PARP) protein, indicating that the growth inhibition was due to apoptosis. CONCLUSIONS: The novel Notch1 activator chrysin inhibits tumor growth in ATC both in vitro and in vivo. Chrysin could be a promising therapeutic candidate for ATC, and this justifies further clinical studies. © 2012 American Cancer Society.