This paper considers models of the variable incidence of health outcom
es in geographical areas and of variable regression effects of socio-e
conomic variables on such outcomes. It adopts a Bayesian approach to v
ariation in relative risk and regression effects, and assesses differe
nt prior specifications of risk (e.g. a latent class structure versus
a spatially correlated structure). Implications are considered for smo
othing and mapping rare health outcomes. The analysis is for electoral
wards in London, with the health-deprivation link forming the focus f
or regression effects. Implications for inferences about risk factors
and for health-need ratings (before and after smoothing) are also cons
idered.