London has the lowest cervical screening coverage in England and in 1998/1999 accounted for 11 of the 13 health authorities that fell below the national 80% coverage target. There are several factors which may contribute to the difference in coverage between the capital and the rest of the country. London's population is much more diverse, there is greater deprivation and there are well-established structural differences in primary care. London has high levels of population mobility which will also affect the ability of GPs to achieve high population coverage. This paper explores the possible size of the effect that population mobility is likely to have on coverage of the cervical screening programme in London. The analysis estimates the size of 'missing populations' that may not receive an invitation for a smear test, or artificially inflate the list size of registered patients. A simple model suggests that in some London Health Authorities up to 14% of residents, and 11% of patients on GP lists, may miss out on invitations for screening as a result of population mobility. Moreover the large differences between list and resident populations in some areas mean that the current government target of 80% coverage of the registered population will be largely unattainable for many London Health Authorities and Primary Care Trusts. Moving towards a resident-based system, whereby the numbers screened are related to the number of residents, avoids some of the problems associated with list inflation and gives a fairer picture of coverage of the eligible population.