Werner syndrome (WS) is an autosomal recessive genetic disorder causing premature aging, and WRN has been identified as the causative gene of WS. The product of the WRN gene (WRN) acts as a DNA helicase with exonuclease activity, and data have accumulated showing that the WRN gene strongly participates in carcinogenesis: (1) the normal WRN gene likely participates in the immortalization of B-lymphoblastoid cell lines through telomeric crisis caused by telomere shortening, (2) a much higher incidence of rare cancers occurs in WS patients than in other kinds of patients, and (3) levels of WRN expressed in virus-transformed cells and cancer cells are usually markedly up-regulated and are inversely correlated with the sensitivity of these cells against various genotoxins, including camptothecin. In this paper, we review the events that show a close correlation of the WRN gene and WRN with carcinogenesis and their underlying molecular mechanisms. (Cancer Sci 2008; 99: 843–848)