Lynch syndrome (LS) here is defined as carriership of a deleterious mismatch repair (MMR) gene mutation. By screening for MMR gene mutations in unselected colorectal or endometrial cancer patients, it was found that the prevalence of LS in colorectal and endometrial cancer patients is 1-3%. On extrapolation to the entire population, the incidence of LS is between 1:2000 and 1:660. As all screening methods are less than 100% sensitive, the above figures are underestimates.