Design-Poisson regression analysis of incidence rates in relation to the variables of interest.

Setting-The 403 county districts of England and Wales during 1979-85.

Subjects-Children aged under 15 years.

Results-There were significant trends in the incidence of lymphoblastic leukaemia at ages 0-4 and 5-9 years with the proportion of children in a district who had recently entered the district. While there was no consistent relation between the proportion of recent incomers in the total population of a district and its incidence rate, the combination of higher migration with greater diversity of origins or distance moved was associated with higher incidence in both age groups. Incidence increased significantly at age 0-4 with the level of employment in a district and at age 5-9 with the proportion of households with access to a car. No significant trends were found with commuting.

Conclusions-The results for level of child migration and diversity of total migration provide evidence of an effect of population mi ring on the incidence of childhood leukaemia which is not restricted to areas experiencing the most extreme levels of mixing.