Many virus and bacteria can cause encephalitis but are rarely identified as the aetiological agent by individual diagnosis. In France, the only continuous source of information about encephalitis is the national hospital medical database (NHMD). Data from the VIH-negative patients recorded in mainland France between 2000 and 2002 with a diagnosis of encephalitis were extracted and analysed according to demographic, geographical and temporal distribution. Hospitalisation details were described. An average of 1200 patients was recorded each year. They were residents of all French districts and equally hospitalized in university hospitals and non university hospitals. Their mean age was 38, and most were men. The aetiological diagnosis was unknown for 80%. The most frequent aetiological diagnosis was herpes simplex virus in adults, and VZV virus in children. These results give us some clues to design a national study on encephalitis. The study will be implemented in mainland France in 2007 and will last one year. We invite all voluntary hospitals to include their encephalitic patients in our study.