Childhood leukaemia clustering--fact or artefact?

There is a general belief that the clustering of childhood leukaemia is a widespread phenomenon and that it provides evidence for appreciable environmental influence on the incidence of the disease. We discuss this issue critically, identifying different kinds of clustering and their possible aetiological mechanisms and examining some analyses of British data. We argue that, in some cases, analyses have been used which lead to dubious conclusions, and that, allowing for multiple testing and anecdotal reporting, the total evidence for clustering is at best weak.