BACKGROUND: Elderly patients are particularly vulnerable to inappropriate prescribing, with increased risk of adverse drug reactions and consequently higher rates of morbidity and mortality. A large proportion of inappropriate prescribing is preventable by adherence to prescribing guidelines, suitable monitoring and regular medication review. As a result, screening tools have been developed to help clinicians improve their prescribing. OBJECTIVES: To compare identification rates of inappropriate prescribing in elderly patients in primary care using two validated screening tools: Beers' criteria and improved prescribing in the elderly tool (IPET); to calculate the net ingredient cost (NIC) per month (euro) of the potentially inappropriate medicines in this population of patients. METHOD: A consecutive cohort of 500 patients 65 years of age and over were recruited prospectively from primary care over a 6 month period in a provincial town in Ireland. Patients' medical records (electronic and paper) were screened and all relevant information concerning current illnesses and medications was recorded on a standardized data collection form to which Beers' criteria [considering diagnosis (CD) and independent of diagnosis (ID)] and IPET tools were applied. The NIC was calculated from an edition of the Irish monthly index of medical specialities published concurrently with the data collection. RESULTS: Beers' criteria identified a total of 69 medicines that were prescribed inappropriately (eight CD and 61 ID) in 65 patients (13%), costing euro824.88 per month while IPET identified 63 potentially inappropriate medicines in 52 (10.4%) patients costing euro381.28 per month. CONCLUSIONS: Potentially inappropriate medications are prescribed in a significant proportion of elderly people in primary care, with significant economic implications.