The UK National Health Service introduced a pay for performance scheme for primary care providers in 2004/5. The scheme rewarded providers for the proportion of eligible patients who received appropriate treatment. Eligible patients were those who had been reported by the provider as having the relevant disease minus those they exception reported as not suitable for treatment. Using rich provider level data, we find that differences in reported disease rates between providers, and differences in exception rates both between and within providers, suggest gaming. Faced with ratio performance indicators, providers acted on denominators as well as numerators.