The association between the risk of childhood leukaemia before age 7 years and population-mixing at the place of residence at birth was investigated by retrospectively considering all the children born in mainland French communes between 1st January 1990 and 31st December 1998. An increased risk of acute lymphoblastic leukaemia was found with higher levels of migration for children residing at birth in isolated communes with a population density > or =50 people per km2 (SIRR = 2.59, 95% CI: 1.48-4.49). No association was observed with lower population densities. For children residing in non-isolated communes at birth, the results were similar but less marked. The risk tended to increase only for population densities > or =5000 people per km2 (SIRR = 1.57, 95% CI: 0.99-2.52). The findings are consistent with epidemic models and support the hypothesis of an infectious aetiology relating to population-mixing. Population density may be seen as an indicator of the opportunity of contacts between inhabitants and should therefore be taken into account when investigating an infectious hypothesis. This is the first systematic study of population-mixing at the place of residence at the time of birth to be conducted on a national scale.