Creutzfeldt-Jakob disease (CJD) is a rare and fatal dementing illness. A direct link has been proposed between cattle infected with bovine spongiform encephalopathy (BSE) and a newly-identified variant of CJD. One possible explanation for the emergence of this new variant, together with a general increase in death due to classical CJD, would be ascertainment bias, due to an increase in the frequency of post-mortems in death attributed to dementia. This article uses national mortality records to look at trends in the proportion of deaths due to dementing illnesses for which a post-mortems was carried out. The results show an increase in deaths due to both CJD and other dementing illnesses, but a slight decrease in the proportion of post-mortems. We conclude that an increase in post-mortems is unlikely to explain the increase in deaths certified as due to CJD. Similarly, the appearance of the new variant of CJD since 1994 cannot easily be attributed to this form of ascertainment bias, in line with the conclusion reached by the Spongiform Encephalopathy Advisory Committee in March 1996.