Viral infection modulates the regulation of apoptosis in host cells. Here, we report a novel mechanism by which human cells infected with mumps virus become susceptible to apoptosis caused by extracellular stresses. Mumps virus stimulates proteasome-dependent degradation of STAT-1 by action of viral accessory protein V, resulting in a severe decrease in STAT-1 protein in infected cells. We exposed mumps virus-infected and uninfected cells to heat and chemical stress. The infected cells failed to acquire resistance to apoptotic stimuli (thermotolerance) after exposure to these mild stresses. The induction of HSP27 by stress exposure was dramatically suppressed in the infected cells, but HSP70 induction was not affected. STAT-1 was required for transcriptional activation of the HSP27 gene, but not for the HSP70 gene, and cDNA transfection of STAT-1 in mumps virus-infected cells restored thermotolerance. Phosphorylated heat shock factor-1 (HSF-1) and STAT-1 phosphorylated on neither tyrosine nor serine residues were co-transported to the nucleus in response to stress. Furthermore, overexpression of unphosphorylatable mutants of STAT-1 also restored thermotolerance in mumps virus-infected cells. These lines of evidence indicate that the induction of HSP27 by stress requires STAT-1 in addition to the activated HSF-1. Furthermore, STAT-1 required for the induction of HSP27 worked independent to its phosphorylation. Thus, HSP27-dependent thermotolerance is suppressed by mumps virus infection through the destruction of STAT-1. The lack of thermotolerance should allow the infected cells to be eliminated by apoptosis and might be a host defense against viral infection.