Objectives: Surveillance of communicable infectious diseases requires close collaboration among clinicians, epidemiologists, and biologists to prevent any risk of an epidemic, particularly following the emergence or unusual incidence of antigenic variants. Meningococcal infections constitute medical emergencies due both to their potentially fatal course for the patient and to their high contagiousness. The example of the sudden emergence of clustered cases of Neisseria meningitidis infections of the rare serogroup W135 among pilgrims returning from Mecca and their close contacts in March 2000 illustrates the importance of implementing alert and control procedures using multidisciplinary approaches. Methods: Systemic infections caused by N. meningitidis are notifiable diseases. Bacterial strains are identified and serogrouped by various clinical laboratories, then sent to the National Reference Center for Meningococci (CNRM) for confirmation and comprehensive antigenic typing. If culture fails, the etiological diagnosis can be obtained through gene amplification directly from samples of cerebrospinal fluid, blood, synovial or pericardial fluid, skin biopsy, etc. When clusters of cases occur, the CNRM and the World Health Organization Collaborating Center perform comprehensive molecular typing. Any suspected outbreak is reported to the Health Surveillance Office of the Directorate General of Health (DGS) and to the Institute for Health Surveillance (InVS). Results Twenty-seven cases of systemic infections caused by N. meningitidis serogroup W135, serotype 2a, serosubtype P1-2.5 were diagnosed between March 22 and November 20, 2000. Molecular typing of the strains showed that they belonged to a single clone related to the ET-37 clonal complex. The spread of a N. meningitidis W135 clone, linked to the return of pilgrims from Mecca who had been vaccinated against meningococcal serogroups A and C, raised concerns about the selection of a new genotype through alteration of the capsule genes. However, strains of meningococcus of the W135:2a:P1.2,5 serotype, which are related to the ET-37 clonal complex, have been isolated in France since 1994. Conclusion: The global spread of this clone raises concerns about the emergence of epidemics, particularly in Africa, and new vaccination measures will need to be implemented.