There are large variations in reported diabetes prevalence within United Kingdom (UK) populations. Linear regression was used to investigate whether population characteristics could explain the variation in prevalence between 19 practices with relatively complete diagnostic recording. Population obesity and South Asian ethnicity largely explained observed variation in prevalence (adjusted R2 = 0.80). When adjusted for obesity and ethnicity, the deprivation score was no longer a predictor of diabetes prevalence. If true variation in prevalence between practices is largely predicted by population ethnicity and obesity, these population characteristics could be used to predict expected prevalence and to assess the completeness of practice registers.