Approaches to investigating clusters: by example, childhood leukaemia near Sellafield.

There is occasionally concern about suspected clusters of disease in geographically localised areas among members of the general public or other groups. These occurrences need a systematic approach to their investigation. Such an approach needs to consider the definition and characteristics of the cluster and of the putative environmental cause for concern. Depending on the outcome of these considerations there will be different actions, possibly involving epidemiological studies. The necessary components of the approach and resulting actions are outlined in this paper, and illustrated by the example of childhood leukaemia rates around the Sellafield nuclear plant.