The clinical phenotype of Werner Syndrome (WRN) includes features reminiscent of accelerated aging and an increased incidence of sarcomas and other tumors of mesenchymal origin. This syndrome results from mutations in the WRN DNA helicase/exonuclease gene. We found that WRN deficient primary fibroblasts, as well as lymphoblastoid cell lines (LCLs), show reduced proliferative survival in response to 4-nitroquinoline-N-oxide (4NQO) and 8-methoxypsoralen (8MOP), compared with WRN-proficient cells. This is the first demonstration of drug hypersensitivity in primary cells of mesenchymal origin from WRN patients. Notably, 8MOP-induced DNA interstrand crosslinks, but not 8MOP mono-adducts, produced S-phase apoptosis in WRN-deficient LCLs. In contrast, 8MOP did not induce S-phase apoptosis in WRN-deficient diploid fibroblasts, in which drug hypersensitivity was entirely due to reduced cell proliferation. Such reduced proliferation of damaged mesenchymal cells in WRN patients may lead to earlier proliferative senescence. In addition, failure of WRN-deficient mesenchymal cells to undergo apoptosis in response to DNA damage in S-phase may promote genomic instability and could help clarify the increased risk of sarcoma in WRN patients. Because interstrand crosslinks are believed to be repaired through homologous recombination, these results suggest an important role for WRN in recombinational resolution of stalled replication forks.