Common but weakly penetrant mutations of certain genes may confer an increased susceptibility to colorectal cancer and account for a proportion of 'sporadic' cases. We analysed DNA from 111 colorectal cancer cases and 114 controls for a specific candidate sequence variation in the hereditary non-polyposis colorectal cancer gene hMSH2. The variant sequence was found in a quarter of individuals, and there was no difference between cancer cases and controls, according to age of development of cancer or presence of family history. It thus appears that this particular sequence variation is a polymorphism rather than a mutation which increases cancer susceptibility.