Classical Hodgkin lymphoma (cHL) is a distinct malignancy of the immune system. Despite the progress made in the understanding of the biology of cHL, the transforming events remain to be elucidated. Recently, we demonstrated that the Janus kinase inhibitor AG490 blocked cellular proliferation and STAT3 phosphorylation in cHL. To explore the potential of constitutively activated STAT3 as a drug target and its role in cHL pathogenesis, different cHL cell lines were analyzed. Treatment of cHL cells by the protein tyrosine kinase inhibitor AG17 was associated with inhibition of cellular proliferation and cell cycle arrest. AG17 treatment was accompanied by decreased levels of STAT3 phosphorylation, whereas NF-kappaB and p38/SAPK2 signaling were not inhibited. Incubation with AG17 or AG490 sensitized cHL cells to CD95/Fas/Apo-1 or staurosporine-mediated apoptosis. Coincubation of tyrphostins with staurosporine was accompanied by rapid complete inhibition of STAT3 phosphorylation. RNA interference directed against STAT3 in L428 and L1236 cHL cells demonstrated that STAT3 is essential for cell proliferation of these cHL cells. In conclusion, these findings support the concept that STAT3 signaling is important in the pathogenesis of cHL and tyrphostins are agents for developing new therapeutic strategies.Leukemia (2005) 19, 936-944. doi:10.1038/sj.leu.2403750 Published online 28 April 2005