ObjectivesTo measure the diagnostic agreement between Primary Care (PC) and hospital information systems, in order to assess the usefulness of health care records for research purposes. SettingCross-sectional retrospective study integrating PC and hospital diagnostic information for the Aragon population admitted to hospital in 2010. Participants75.176 patients were analysed. InterventionsSimilarities, differences and the kappa index were calculated for each of the diagnoses recorded in both information systems. Main measurementsThe studied diseases included COPD, diabetes, hypertension, cerebrovascular disease, ischaemic heart disease, asthma, epilepsy, and heart failure. ResultsDiagnostic concordance was higher in men and between 45 and 64 years. Diabetes was the condition showing the highest concordance (kappa index: 0.75), while asthma had the lowest values (kappa index: 0.34). ConclusionsThe low concordance between the diagnostic information recorded in PC and in the hospital setting calls for urgent measures to ensure that healthcare professionals have a comprehensive picture of patient's health problems.