There are many secondary benefits to collecting routine primary care data, but we first need to understand some of the properties of this data. In this paper we describe the method used to assess the PPV and sensitivity of data extracted from Belgian GPs' EPR (diagnoses, drug prescriptions, referrals, and certain parameters), using data collected through an electronic questionnaire as a gold standard. We describe the results of the ResoPrim phase 2 project, which involved 4 software systems and 43 practices (10,307 patients). This method of assessment could also be applied to other research networks.