In January 1998, the Departmental Directorate of Health and Social Affairs (DDASS) of Seine-Maritime reported to the National Public Health Network a significant increase in the number of meningococcal infections (MI) reported in the department in 1997. At the same time, the National Reference Center for Meningococci and Related Neisseria (CNRM) reported the presence in patients from Seine-Maritime of a strain rarely isolated in France and belonging to the ET-5 electrophoretic complex, strain B:14:P1.7,16. In this context, it became necessary to conduct a detailed analysis of the available epidemiological data. The number of cases reported in Seine-Maritime in 1997 far exceeded the expected number compared to previous years and compared to other departments. The isolation, in nearly half of the cases, of a strain from the ET-5 complex responsible for an epidemic in Northern Europe is a concerning phenomenon that could explain part of the observed excess of cases. There is a situation of persistent hyperendemicity in the department of Seine-Maritime.