Deficiencies in mismatch repair have been linked to a common cancer predisposition syndrome in humans, hereditary nonpolyposis colorectal cancer (HNPCC), and a subset of sporadic cancers. Here, several mismatch repair-deficient tumor cell lines and HNPCC-derived lymphoblastoid cell lines were found to be deficient in an additional DNA repair process termed transcription-coupled repair (TCR). The TCR defect was corrected in a mutant cell line whose mismatch repair deficiency had been corrected by chromosome transfer. Thus, the connection between excision repair and mismatch repair previously described in Escherichia coli extends to humans. These results imply that deficiencies in TCR and exposure to carcinogens present in the environment may contribute to the etiology of tumors associated with genetic defects in mismatch repair.