An important obstacle in assessing the real burden of road accident injuries in Greece and Europe is under-reporting of non-fatal casualties. On that purpose, eight national studies were carried out using a common methodology and allowed to prepare valid estimates of the level of under-reporting of non-fatal road casualties for each country. Τhe Greek study, aimed to (1) assess the magnitude of road traffic injuries in a country missing a formal linkage system of police with hospital data, (2) quantify the under-reporting, and (3) produce a convenient algorithm exploring its constituent components. Linkage of disaggregate data collected by the Road Traffic Police (RTP) with those by the Emergency Department Injury Surveillance System (EDISS) on the Greek island of Corfu was carried out. The applied four-step methodology led to the identification of the overall under-reporting from either registry. Road Traffic Police data captured 96.6%, whereas EDISS only 54.4% of total fatalities (overall concordance: 51.1%). On the contrary, EDISS captured 94.6% of non fatal injuries, whereas RTP only 16%, resulting in a low overall concordance (10.6%). Considering severity of injury assessed by EDISS as the gold standard, RTP data misclassified 45.7% of severe injuries as less severe.